The Hoover UH70400 belongs to the WindTunnel Air serials whereas the UH70210 is one of the Pet models of the T-Series WindTunnel serials.
The difference between Hoover UH70400 and UH70210 are:
UH70210 vs UH70400
images from manufacturer

Cyclonic Action Air System:
The UH70210 features Dual Cyclonic Air System while the UH70400 features a Multi-Cylonic Action.

Tools:
The UH70210 is a pet model, it comes with Air Driven Pet Hand Tool and Pet Upholstery Tool with rubber blades to help remove pet hair from furniture and upholstery, whereas the UH70400 includes a Upholstery/Dusting Brush combo tool, but does not have the air powered tool, it also comes with a 3-in-1 Combination Tool, and a Crevice Tool. The UH70210 also has a Crevice Tool.

Filtration:
The UH70210 equips with Carbon Filter to absorb pet odors whereas the UH70400 does not feature the Carbon Filter.

Motor Amps:
The UH70400 equips with 11 amps motor whereas the UH70210 has 12 amps.

Height Adjustment:
The UH70400 uses automatic height adjustment to fit the floors you vacuum whereas the UH70210 uses manual height setting, there are five options to select.

Brushroll Switch Location:
The brushroll switch of Hoover UH70400 is on the top of the vacuum body whereas the UH70210 equips with a foot pedal at the left side of its base to switch the brushroll.

Nozzle Width: The nozzle width on UH70210 is wider.
The UH70400 has a relative small nozzle, its width is just 11-inch while the UH70210 has 13.5 inch nozzle width.

Cord Rewind:
There is a cord rewind pedal on the UH70210 that you can tap to retract the cord automatically whereas the UH70400 does not feature auto cord rewind.

Cord Length: The UH70400 has a longer cable.
The cord on UH70400 is 30 ft long whereas the cord length of UH70210 is 25 ft long.

Color:
The UH70400 is grey with a little green whereas the UH70210 is blue.

Weight: The UH70400 is over 5 pounds lighter.
The UH70400 has a weight of 12.4 pounds whereas the UH70210 weighs 17.8 pounds.

Headlight:
The UH70400 has no headlamp for dark spot cleaning whereas the UH70210 features one.

Power ON/OFF Switch Location:
The ON/OFF Switch on the UH70210 is on the handle while the UH70400 has it on the top of the vacuum, next to the brushroll switch.

Deep Recline:
The Deep Recline feature allows the vacuum to recline flat to get under furniture. The UH70400 features it whereas the UH70210 does not.

Handle:
The Hoover UH70400 features an adjustable handle with built-in wand that you can remove for cleaning hard to reach places, the handle cannot be folded, whereas the UH70210 has a foldable handle that can be folded for compact storage, but it is not adjustable, and its wand 12 inch extension wand is not built-in type.

Hose:
The UH70400 includes a Quick Fit Hose and a 8 ft Stair Hose whereas UH70210 has a 8 ft long stretch hose.

Price: The UH70400 is $30 cheaper than the UH70210.
The UH70400 is priced at $129.99 at hoover.com whereas the UH70210 costs $159.99. (prices checked on June 19, 2013)

To learn details, download manuals:
UH70400 Manual: http://hoover.com/file/578/
UH70210 Manual: http://hoover.com/file/574/

Please share below if you know more difference between Hoover UH70400 and UH70210.