You've probably made up your mind by now, but for anyone else in the same quandry...
I own an '08 STi and have driven the new WRX's. Both are awesome. The STi is of course faster (other than 0-60 because of extra shift) in the real world, but most importantly, the handling is really a couple of notches better. If you're going to drive really hard, the STi gearbox etc will stand up better, and the brakes are amazing.
Having said all that, I would still go with a new car (WRX) when it comes to sports cars, especially Turbos. You don't know how the previous owner treated the car; did he warm it up fully before pushing it? Did she cool down the turbo especially after a hard run? Abuse the clutch for stop-light racing?
For these reasons, I'd go with the new car in this kind of sports car. Either way, do what I did and you'll be very happy! (Replace stock DP with catted high flow, Cobb AP, Sway Bars and link). With those modest changes, either car will be transformed to a BEAST and yet still be reliable and civilized. My car was dyno'd at 360hp and just over 400lbs torque with just those changes (without even the rest of the exhaust yet - so it's still very quiet).
So, unless you're tracking or really driving hard a lot, go with the new WRX and do the mods, you won't be sorry - it will make one heck of a DD.
I only went up to the STi because of the crazy sale what was going on for the '08's - I paid 29K plus tax for mine. Without that deal, the WRX is a better price/performance ratio and will probably give you better mileage to boot.
K, that;s more than enough!