The sti is more of a brute performer. It will toss you around a bit more, wont be as nice over bumps, and will sit you back in your seat. The wrx is an decent powered entry level sedan with medicore refinement and decent handling.
Neither of the two cars are expensive to mod at all, depending on what your goals are. The stil can very easily be a 12 second car for a few (2-4) grand.
As far as reliabilty people are just now starting to report that the 04's sti's are hitting high miles. Most of them are doing fine (menus a rear strut issue alot complain about...probaly fixed in later models). Several wrx's have gone over 100,000 with very little issues.
To end this.....2-3 years from now who knows what the wrx will be, it is way to early to plan ahead. I joined in 03 and the car has come leep and bounds since then.