Get the newest car that you can afford with the lowest miles possible because you will probably become attached to the car and keep it for a long time. Most people prefer 2009 model WRXs over 2008 models because of the visual aspect of the wider/flared fenders and the more powerful VF-52 turbo. 2008 models had flatter fenders and the same (less powerful) TD-04 turbo that came on WRXs prior to 2009.
Used (unmodified) STis hold their value rather well, so you'll pay significantly more for one than a used WRX of similar age and mileage. With a budget of roughly $15k, I'd probably steer you toward a WRX. It might help if you can narrow your search down to a few specific cars and we can give you advice and opinions on each vehicle.
A golden rule for used car buying is to get one that's as close to stock as possible. Some mods aren't really a big deal, but others raise red flags.
Welcome to the forum, by the way!
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)