The difference between Hoover UH30300 and UH30310 are:
Hoover UH30310 vs UH30300
images from manufacturer

Standard VS Pet:
The Hoover UH30300 is a standard model while the UH30310 is pet model.
Pet model features Activated Carbon to absorb pet odors and other household odors whereas the standard model does not have.
Pet models have tools that are designed for pets, the air driven pet hand tool and the pet upholstery tool feature rubber wipers to help remove pet hair from furniture and upholstery. While the tools on standard models do not feature rubber wipers.

Hose: The UH30310 has a longer stretch hose.
The UH30300 comes with a 8 ft stretch hose whereas the UH30310 has a 12 ft deluxe stretch hose.

Price: The UH30310 is $10 more expensive than the UH30300.
The UH30310 is priced at $149.99 at hoover.com whereas the UH30300 is priced at $139.99 (prices checked on July 7, 2013)

Other than that they are almost the same.

They are T-Series Windtunnel bagged upright vacuum cleaners.
They feature Brushroll Shutoff that allows you to use it on bare floors without the risk of damaging the floor by the spinning brush.
Equip with Headlight to illuminate dark areas for thorough cleaning.
Full Bag Indicator to tell you when to check bag and filter.
5 Level Height Adjustment that allows you to adjust to clean different flooring types.
Feature D-Shaped Handle.
Weighs about 15 pounds.
Equip with 12 amp motor.
Have a 30-ft cord.
13.5 nozzle width.
HEPA media filters traps more than 99.9% of tiny particles down to 0.3 microns.
No cord rewind.

You can learn more by downloading the manuals:

UH30300 manual
http://hoover.com/file/543/

UH30310 manual
http://hoover.com/file/544/

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