is this a complete from scratch setup?
Basically it is 50 ap's per switch, 200 per stack for the 3750.. you'll have more redundancy by building multiple stacks in your closets.
WISM = money.. not for the card per se, but I believe it require the sup720, full chassis 6500 etc.
plus you'd want to separate them for redundancy so we're talking two chassis geographically disperse... which if your going to do that, you might as well buy the 3750's.
The wism's handle 300 ap's so to me I'd want full platform redundancy. I would hate to see 250 ap's just die off the network cause a box died or lost power.
the only caveat is I'd talk with your cisco sales rep, 3750's having been out awhile, and see if the lightweight ap support is going to be another product in the 3750-E line.
there are design considerations also, if you want to do NAC or something like that, the wism's would terminate all the traffic more centrally, or if your not concerned with that, you could run reap on your access points (provided you get the right ones) and distribute traffic closer to the ap edge point with the 3750's and reap, your traffic would be far less centralized. Depends if your AP's are geographically dispersed (ie at branch offices) or on one campus with fast backbones.