I just wanted to add - I picked FWP when faced with that choice.
Aside from what has already been discussed, IMO FWP is unlike any back they have had in Pittsburg for close to.....well, ever. He was enough of a special back for the team to overhaul last years offense (Look for more of a no-huddle wide open attack) to spread the field for FWP. I like FWPs potential this year assuming they can keep his touches in the 300-330 range. Any more and he could be an injury risk also (NFL = Not For Long). Hence, IMO why they choose to keep 4 Rbs. IMO that Steelers team has 500-525 carries (including actual Playoffs and Super Bowl) in it if it is any good. (425-450 for the season).
Westbrook, on the other hand, is a singular talent that cannot stay healthy. It has to be noted though that this is the first offseason that wasn't a short offseason for that team. They had gone to the Championship game 4 years in a row before last year. Rest will do a body good, it's just so hard to say with Westbrook whether rest was a good thing or a bad one.
Westbrook was the top, if not the 2nd best, offensive option this team has had in that time. To not recognize that the Eagles are not only a McNabb led team but a Westbrook led team at this point (as far as any hope of Championships are concerned) would be a mistake in estimating his value. Coach Reid also is aware of all of this and will protect his player to the best of his ability.
Having said this I shied away from Westbrook based on a tougher strength of schedule (by a hair-but especially at PO time) and the injury concern. I picked Willie because they changed the Offense for him and no one should see him coming yet.