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.