the WRX is a great car and a ton of fun to drive. plenty of power on tap in stock form to make it enjoyable, and it takes pretty well to basic bolt on mods. 280 awhp is relatively common on these cars with upgraded turbo and intercooler, exhaust, and other supporting mods. more than 300 awhp is more difficult to attain. you do need to be somewhat gentle on the tranny though unless you've budgeted for an upgraded one. $16k for the one you described seems like a pretty good price for it.
the STi is well worth the extra cost, although i think you might be able to do better than $28k for a used one. you probably know more about the used market than i do though. this car has a substantial amount more power from the factory, and 350 awhp is very realistic with just bolt ons. the tranny is a much better and stronger 6 speed, and there are plenty of other goodies like BBS wheels, Brembo brakes, STi suspension, DCCD, and plenty more.
if you can manage to get the STi without putting yourself into an uncomfortable financial situation, you'll never regret it. if its just not attainable at this point, you can get a WRX and be very happy with it, and keep yourself busy with wishing you had an STi (thats what i do ).
go drive both and your heart will lead the way