Engine wise the two bikes are nearly identical, although I believe that the WR450F has slightly different carburetor jetting to meet EPA requirements. (I also understand that there is a kit to undo this and make it identical to the YZ). The WR also has an electronic rev limiter which, if you feel the need, can be simply disconnected ("grey wire modification").
The WR450F has a wider ratio transmission, which may or may not be of interest to you. It is possible to make the WR450 street legal in some states so if you wanted to ride it on the street the "faster" high gear would definately come in handy.
Suspension wise the two bikes are very close, but the WR450 has an 18 inch rear wheel instead of the 19 inch that the YZ has. There is a slight difference in rear suspension travel (12.0" vs 12.4").
The WR450 has a slightly larger gas tank, which can come in handy for long rides.
The seat height is ~3/4" lower.
The biggest difference is that the WR450 weighs ~30 pounds more. This is primarialy due to the battery, electric start, kickstand and headlight.
For reasons I don't understand the WR450 is a little bit cheaper (MSRP) than the YZ.
My next bike is going to have a kickstand, no doubt about it. I am really starting to think that electric start is a good idea and worth the extra weight.
What it really comes down to is the extra 30 pounds and a wide ratio transmission. If you were going to race or tackle really gnarly hills where a close ratio transmission would keep you up in the power curve then I would recommend the YZ. For just general ease of use and overal fucntion I would recommend the WR.