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.