The difference between Hoover UH70400 and UH70600 are:

UH70600 vs UH70400
images from manufacturer

Cord Rewind:
The UH70600 features auto cord rewind that you can touch the pedal to retract the cord automatically without winding manually, and the cord stores neatly inside, while the UH70400 does not equip with the cord reel, its cord should be wrapped around the back side.

Cord Length: The cord on UH70600 is shorter.
The UH70400 has a 30 ft cable while the cord length of UH70600 is 28 ft long.

Weight: The UH70600 is about 6 pounds heavier.
The UH70400 weighs only 12.4 lbs while the UH70600 weighs 18.4 lbs. If you need a lightweight upright, then the UH70400 should be a better choice.

Nozzle Width: The UH70600 has wider nozzle width.
The UH70400 has nozzle width of 11-inch while the UH70600 has 15-inch nozzle width.

Motor Amps:
The UH70400 has 11 amps motor while the UH70600 has 12 amps.

Height Adjustment:
The UH70400 equips with automatic height adjustment whereas the UH70600 features manual height adjustment that has 7 settings for different floor types.

Headlight:
The UH70400 does not have headlight whereas the UH70600 equips with one for cleaning dark areas of the room.

Beater Bar ON/OFF Placement:
The brush ON/OFF control of Hoover UH70400 is located on the top of the vacuum body while the UH70600 equips with a foot pedal at the base to control the brush.

Power Button Placement:
The power button on the UH70600 is on the handle whereas the power button on UH70400 is located on the top of the vacuum unit, next to the brush ON/OFF button.

Tools:
The UH70400 includes a 3-in-1 combination tool, a Crevice Tool, a combo Upholstery/Dusting Brush, 30″ Extension Wand.
While the UH70600 comes with: a 10″ Crevice Tool, a 2-in-1 Upholstery/Dusting Brush, 20″ Extension Wand.

Deep Recline Handle:
The UH70400 equips with a Deep Recline Handle that allows the unit recline flat to clean under furniture whereas the UH70600 does not feature this.

Hose:
The UH70400 comes with a Quick Fit Hose and a 8 ft Stair Hose while the UH70600 has a 15 ft hose.

Price:
The UH70400 is priced at $129.99 at hoover.com whereas the UH70600 costs $199.99.

Color:
The UH70400 is grey with a little green whereas the UH70600 is blue.

Handle:
The Hoover UH70400 equips with a telescopic handle with built-in wand that is detachable, whereas the handle on UH70600 is not extendable and its wand is not built-in.

Please share below if you know more difference between Hoover UH70400 and UH70600.