WRX's really suck on highways, at highway speeds acceleration is really slow.
But for winter I'd rather have WRX, for summer M3. Overall, Id take M3.
0-100km/h sti is close to the e46 m3 and it is a drive race, wrx is close to the e36 and it is a driver race. Athigher speed m3s will pull.
Im not really a fan of both WRX's looks, so I would take M3, but for the money you get alot from WRX.