Here are difference between these 2 hoover carpet washers:
Hoover F7452900 vs F7412900
images from manufacturer

Accessories for Hard Floor Cleaning:
The Hoover F7452900 comes with accessories for cleaning hard floors while the F7412900 does not have.
The accessories include a Hard Floor Squeegee and Hard Floor Brush Assembly. They are intended for hard floors only, not for use on rugs or carpets.
The hard floor brush assembly has white longer bristles whereas the brush for carpets is green and with shorter bristles.

Weight:
The F7412900 hand weight is 27 pounds whereas it is 30 pounds on F7452900.

Price: The F7412900 is cheaper.
The F7412900 is priced at $199.99 whereas the F7452900 costs $219.99 (prices checked on July 22, 2013 at hoover.com).

Similarities:
They have 12 amp motor;
Carries one year limited warranty;
20 ft power cord;
13 inch nozzle width;
8 ft hose;
1 gallon tank capacity;
feature Edge Cleaning Bristles to help clean dirt near baseboards;
dual tanks to keep clean and dirty water separate;
come with SpinScrub Hand Tool;
feature Heated Cleaning to dry the surface;
fingertip ON/OFF switch for quick and convenient control.

Read manuals to learn more:
F7412900 manual: http://hoover.com/file/593/
F7452900 manual: http://hoover.com/file/1088/

If you know more difference between Hoover F7452900 and F7412900, please share here.

Hoover F7452900 vs F5912900
images from manufacturer

Hard Floor Cleaning Package:
The F7452900 comes with hard floor cleaning package to help clean your hard floors, while the F5914900 does not have, includes a hard floor squeegee and a hard floor brush assembly.

Auto Rinse Technology:
The F7452900 features Auto Rinse that rinses your carpet and hard floors, leaving less detergent residue behind.
It has a rinse setting switch that allows you to select a Wash-AutoRinse setting or a Rinse only setting.
When you select Wash-AutoRinse, it cleans your carpet with detergent and water on the forward stroke. On the reverse stroke, a water only rinse is applied automatically.
When you select Rinse, it applies water only rinse in both the forward and reverse stroke.
The F5914900 does not have this feature.

Edge Cleaners:
The F7452900 features edge cleaners to help clean dust near baseboards while the F5914900 does not have the edge cleaning bristles.

SpinScrub:
The F5914900 features SpinScrub 50 whereas the F7452900 features SpinScrub 60.
The SpinScrub 50 has 5 rotating brushes in the nozzle while the SpinScrub 60 has 6. With the sixth brush, SpinScrub 60 provides 20% more cleaning action than the SpinScrub 50, so you clean the carpet and floor faster with the SpinScrub 60.

ON/OFF Control:
The F5914900 features a foot pedal to turn ON/OFF the machine, while the F7452900 has a fingertip ON/OFF switch on the handle.

Nozzle Width:
The F7452900 has 13 inch nozzle whereas the nozzle width on the F5914900 is 11.25 inches.

Weight: The F7452900 is heavier.
The F7452900 weighs 30 pounds while the F5914900 weighs 20 pounds.

Tank Capacity:
Based on the data from hoover.com product pages, the tank capacity on F5914900 is 133 ounces while the F7452900 comes with 128 ounces tanks.

Price: The F5914900 is cheaper.
The F7452900 is priced at $219.99 at hoover.com while the F5914900 costs $169.99 (prices checked on July 20, 2013).

You can learn more by reading manuals:
F5914900 manual: http://hoover.com/file/604/
F7452900 manual: http://hoover.com/file/1088/

If you know more difference between Hoover F7452900 and F5914900, please share below.

Hoover F5914900 vs F7412900
images from manufacturer

SpinScrub:
The F5914900 equips with SpinScrub 50 whereas the F7412900 features SpinScrub 60.
The SpinScrub 60 has six rotating brushes in the nozzle, the sixth brush provides 20% more cleaning action in a single pass than the five-brush model, thus it gets the job done more quickly.

Edge Cleaning:
The F7412900 features edge cleaning bristles on both sides of the nozzle to help remove dirt and debris near baseboards, while the F5914900 has no edge cleaners.

Nozzle Width: The F7412900 has a wider nozzle.
The F5914900 has a nozzle width of 11.25 inches while the F7412900 has 13 inches.

Weight:
The F5914900 weighs 20 pounds while the F7412900 weighs 27 pounds.

Tank Capacity:
The F5914900 has a tank capacity of 133 ounces while it is 128 ounces on F7412900.

Location of ON/OFF Control:
The ON/OFF control on F5914900 is a foot pedal that is located on its left side, while the ON/OFF switch on F7412900 is located on the handle where you can easily get with your fingertips.

Price: The F7412900 is more expensive.
The F5914900 is priced at $169.99 at hoover.com while the F7412900 is priced at $199.99 (prices checked on July 19, 2013).

The similarities:
They have 12-amp motor, 8 ft hose, 20 ft cord, dual tanks, heated cleaning feature to dry faster, SpinScrub Powered Hand Tool for cleaning upholstery and stairs, 3 speed brushroll control, brushes can be detached for rinsing, backed up with one year limited warranty.

You can download manuals to learn more:
F5914900 manual: http://hoover.com/file/604/
F7412900 manual: http://hoover.com/file/593/

If you know more difference between Hoover F7412900 and F5914900, please share here, thank you.