STI has different suspension, brakes, seats, body parts, gearbox, mapping, exhaust, etc and are all generally upgrades from the WRX. Both are excellent cars and, speaking as someone who had an 03 WRX for 3 years, great cars. Fuel economy and general "live-ability" is better with a WRX but you'll end up upgrading some bits and bobs (everyone does) and some of them may be STI type things.
The RB320 is the best of the lot and would be well worth the stretch (and hold it's value best) BUT the STI or GB270 probably offers the best value for money at present.
Brian.