The major difference between UH70140 and UH70120 are:

Brushroll ON/OFF Switch:
The Hoover UH70140 equips with brushroll ON/OFF function, you can step a petal to turn on or shut off the brushroll for suction only mode, whereas the UH70120 has no brush ON/OFF switch.

Height Adjustment:
Both of them equip with the height adjustment, however, there is slight difference. The UH70120 has 5 height settings while the UH70140 has 3 levels to adjust.

Headlight:
The UH70140 has a headlight that allows you to clean areas like dark corners thoroughly whereas the UH70120 have no such headlight.

Air System:
The UH70120 features Dual Cyclonic Air System while the UH70140 uses Multi-Stage Cyclonic Technology.

Color:
The UH70120 is green while the UH70140 is red.

Price: The UH70120 is $20 cheaper than the UH70140.
The UH70120 is priced at $129.99 at hoover.com while the UH70140 is priced at $129.99 (prices checked on June 23, 2013)

Similarities between UH70140 and UH70120:
have auto cord rewind function;
equip with motorized brush;
powered by 12 amps motor;
weighs 16.5 pounds;
8 ft stretch hose;
13.5-inch cleaning path width;
include HEPA filter;
2 year limited warranty;
comes with onboard tools: Turbo Hand Tool, Upholstery/Dusting Brush, Extension Wand, Crevice Tool;
indicator to tell you when to check dust bin and filter;
scuff guard bumper protects your furniture;
fingertip power control.

You can learn more by reading manuals:
UH70120 Manual Download: http://hoover.com/file/570/
UH70140 Manual Download: http://hoover.com/file/588/

Please share below if you know more difference between Hoover UH70120 and UH70140.