Some key difference between Hoover UH70210 and UH70100 is handle design, cord winding mechanism, headlight, and brushroll switch.
** The UH70210 has a folding down handle while the handle on UH70100 is not foldable.
** The UH70210 is equipped with auto cord rewind while the UH70100 does not feature automatic cord retractor.
** The UH70210 has a headlight while the UH70100 does not have.
** The UH70210 features suction only mode while the UH70100 has no brushroll On/Off switch.
More specs comparison, read the table below:

Compare Hoover UH70100 and UH70210

	Hoover UH70210 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70100 T-Series™ WindTunnel® Pet Bagless Upright
Design	Upright	Upright
Price	$159.99 at Hoover.com on August 12, 2014	$119.99 at Hoover.com on August 12, 2014
Bagged or Bagless	Bagless	Bagless
Cord Length	25 ft	27 ft
Auto Cord Rewind	Yes	No
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Suction Only Mode	Yes	No Brushroll cannot be turned off once vacuum is on.
Height Adjustment	5 Manual Settings	5 Manual Height Settings
HEPA Filtration	Yes	Yes
Headlight	Yes	No
Weight	17.8 lbs	15.7 lbs
Cyclonic Action	Dual Cyclonic Air System	Dual Cyclonic Air System
WindTunnel Technology	Yes	Yes
Performance Check Indicator	Yes	Yes
Nozzle Width	13.5"	13.5"
Edge Cleaning Brush	No	No
Bumper Type	Scuff Guard	Soft Touch
Self-propelled	No	No
Grip Type	D-Shaped Handle	Standard Grip Handle
Folding Down Handle	Yes	No
Hose Type	stretch hose	stretch hose
Hose Length	8'	8'
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Pet Upholstery Tool
Cleaning Wand	Yes	Yes
Color	Light Blue	Blue
Warranty	2 year limited warranty	2 year limited warranty
Manual	download manual	download manual

Overall, the UH70210 is a better choice though it costs more.
If you have more info to share on these 2 Hoover upright, please comment below.

The apparent difference between Hoover UH70210 and UH70240 is the Handle design and the color.
** The UH70210 features a folding down handle while the handle on the UH70240 is not foldable.
** Primary color of the UH70210 is light blue while the blue color on the UH70240 is darker.
** The UH70210 features Dual Cyclonic Air System while the UH70240 features Multi-Stage Cyclonic Technology.
They both have the retractable cord and suction only feature.

Compare Hoover UH70240 and UH70210

	Hoover UH70210 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70240 WindTunnel® T-Series™ Pet Rewind Bagless Upright
Design	Upright	Upright
Price	$159.99 at Hoover.com on August 12, 2014	$127.49 at Hoover.com on August 12, 2014
Bagged or Bagless	Bagless	Bagless
Cord Length	25 ft	27 ft
Auto Cord Rewind	Yes	Yes
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Suction Only Mode	Yes	Yes
Height Adjustment	5 Manual Settings	5 Manual Height Settings
HEPA Filtration	Yes	Yes
Headlight	Yes	Yes
Weight	17.8 lbs	not specified
Cyclonic Action	Dual Cyclonic Air System	Multi-Stage Cyclonic Technology
WindTunnel Technology	Yes	Yes
Performance Check Indicator	Yes	Yes
Nozzle Width	13.5"	13.5"
Edge Cleaning Brush	No	No
Bumper Type	Scuff Guard	Soft Touch
Self-propelled	No	No
Folding Down Handle	Yes	No
Hose Type	stretch hose	stretch hose
Hose Length	8'	not specified
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool
Cleaning Wand	Yes	Yes
Color	Light Blue	Blue
Warranty	2 year limited warranty	2 year limited warranty
Manual	download manual	download manual

If you know other difference between UH70240 and UH70210, please share it below, thanks.

According to product features and specs on hoover.com, the difference between Hoover UH70210 and UH70212 are the color and the accessories they include.
** You can see that the UH70210 is blue while the primary color of UH70212 is black.
** The UH70212 comes with a deluxe stretch hose that is 12 ft long while the UH70210 include a 8 ft stretch hose.
** The UH70212 also comes with 30 ct. Hoover Pet Stain and Odor Wipes that the UH70210 does not include.

Hoover claim that the UH70212 is equipped with the P.A.W.S. Pet Approved WindTunnel System.
Do not know how this P.A.W.S. differs from the general WindTunnel technology.

Both of them have the brush om/off switch, headlight, and cord rewind system.

Compare Hoover UH70212 and UH70210

	Hoover UH70210 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70212 T-Series™ WindTunnel® P.A.W.S. Pet Rewind Bagless Upright
Design	Upright	Upright
Price	$159.99 at Hoover.com on August 12, 2014	$159.99 at Hoover.com on August 12, 2014
Bagged or Bagless	Bagless	Bagless
System Check Indicator	Yes indicates when a clog needs to be removed or a filter needs to be cleaned	Yes
Bottom-Release Bin	Yes	Yes
Cord Length	25 ft	25 ft
Auto Cord Rewind	Yes	Yes
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Brushroll On/Off	Yes	Yes
Height Adjustment Type	Manual, 5 Settings	Manual, 5 levels
HEPA Filtration	Yes	Yes
Headlight	Yes	Yes
Weight	17.8 lbs	17.8 lbs
Cyclonic Action	Dual Cyclonic Air System	Dual Cyclonic Air System
WindTunnel Technology	WindTunnel	Pet Approved WindTunnel® System
Full Bin Indicator	No	No
Nozzle Width	13.5"	13.5"
Edge Cleaning Brush	No	No
Self-propelled	No	No
Handle	Foldable Handle	Foldable Handle
Hose Type	stretch hose	Deluxe Stretch hose
Hose Length	8'	12'
Cleaning Wand	Yes	Yes
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Rotating Pet Power Brush	Crevice Tool, Dusting Brush, Pet Upholstery Tool, Rotating Pet Power Brush Includes 30 ct. Hoover Pet Stain and Odor Wipes
Color	Blue	Black
Warranty	2 year limited warranty	2 year limited warranty
Manual	download manual	download UH70212 manual

If you own Hoover UH70210 or UH70212, and know other difference between them, please share here.

The major difference between Hoover UH70102 and UH70120 is the cord winding mechanism. For the Hoover UH70102, you have to wind the cord manually, while the UH70120 features self winding cord reel, you just tap the pedal and the cord retracts.
Another difference is the Turbo Tool they include. The tool on UH70120 is a common air powered tool that use bristle while the UH70102 come with a Rubberized Pet Turbo Tool that has rubber blades to help pick up pet hair.

Compare Hoover UH70102 and UH70120

	Hoover UH70120 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70102 T-Series™ WindTunnel® Pet Bagless Upright
Design	Upright	Upright
Price	$129.99 at Hoover.com on August 11, 2014	$99.99 at Hoover.com on August 11, 2014
Bagged or Bagless	Bagless	Bagless
Bottom-Release Bin	Yes	Yes
Cord Length	25 ft	27 ft
Retractable Cord	Yes	No
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Brushroll On/Off	No no way to turn off the brush when vacuum is on	No
Height Adjustment Type	Manual, 5 Settings	Manual, 5 Level
HEPA Filtration	Yes	Yes
Headlight	No head light	No
Weight	16.5 lbs	15.7 lbs
Performance Check Indicator	Yes	Yes
Cyclonic Action	Dual	Dual Cyclonic Air System
WindTunnel Technology	WindTunnel	WindTunnel
Nozzle Width	13.5"	13.5"
Self-propelled	No	No
Handle	D-Shaped Handle	D-Shaped Handle
Bumper Type	Scuff Guard	Scuff Guard
Hose	8' stretch hose	8' stretch hose
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	Rubberized Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool
Wand	Yes	Yes
Wand Style	Fixed	Fixed
Color	Gray and Green	Purple
Warranty	2 year limited warranty	2 year limited warranty
Manual	download uh70120 manual

So, no big difference except for the retractable cord.
If you like a vacuum with cord reel, then the UH70120 should meet your needs. By the way, it is a hot selling item on amazon.com. You can read reviews there before buying.

There is almost no difference btween UH70110 and UH70120 except for the color and the tiny design difference on the base. If you look closer, you will find that the back of the base on the UH70120 is slightly wider.
Otherwise, they are identical. Feature 12 Amps motor, cord reel, 8 feet stretch hose, 5 level height adjustment, no brush off switch, and 13.5 inches cleaning path. For more specs, read below.

Compare Hoover UH70110 and UH70120

	Hoover UH70120 WindTunnel T-Series Rewind Plus Bagless Upright Vacuum Cleaner	Hoover UH70110 Bagless Upright
Design	Upright	Upright
Price	$129.99 at Hoover.com on August 9, 2014	$129.99 at Hoover.com on August 9, 2014
Bagged or Bagless	Bagless	Bagless
Dirt Finder	No	No
Bottom-Release Bin	Yes	Yes
Cord Length	27 ft	27 ft
Auto Cord Rewind	Yes	Yes
Amps	12 Amps	12 Amps
Motorized Brushroll	Yes	Yes
Brushroll On/Off	No no way to turn off the brush when vacuum is on	No
Edge Cleaning Brush	No	No
Height Adjustment Type	Manual, 5 Settings, adjust as needed to vacuum different types of floor	Manual, 5 Level
Washable Filter	Yes	Yes
HEPA Filtration	Yes	Yes
Headlight	No head lamp	No head lamp
Weight	16.5 lbs	16.5 lbs
Performance Check Indicator	Yes	Yes
Cyclonic Action	Dual	Dual Cyclonic Air System
WindTunnel Technology	WindTunnel	WindTunnel
Nozzle Width	13.5"	13.5"
Self-propelled	No	No
Handle	D-Shaped Handle	D-Shaped Handle
Deep Recline Handle	No	No
Hose Type	stretch hose	stretch hose
Hose Length	8'	8'
Included Tools	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool	Air Powered Hand Tool, Crevice Tool, Dusting Brush, Upholstery Tool
Cleaning Wand	Yes	Yes
Color	Gray and Green	Light Gold
Warranty	2 year limited warranty	2 year limited warranty
Manual	download manual	download UH 70110 manual

If you know other difference between these 2 hoover upright vacuum with retractable cord, please share below.