Problems with WRX
1. Just as much torque as your stang, but you have to use a blower to get to it.
2. Seats to firm IMO
3. Interior feels cramped IMO
4. Compared to a 99+ the interior quality is not that much better IMO
5. Gas Mileage is not that big of a difference.
6. The rear seats of my stang fold down (don't know about yours) I can move what I need to. The trunk is just as small in a WRX unless you get a wagon.
7. Smaller dealer network, can make parts hard to find, and more expensive
8. More expensive to put tires on, unless you have 17's on your stang
Positives of WRX
AWD really helps out in severe driving conditions
Faster
Hrm, what else...
I don't know, but if you catch me this is all IMO. I disliked ford before I bought my stang, but I got an amazing deal on a low mileage car, and it felt right. I wanted an F-body or a Chrysler 300M (big difference right?). But I couldn't find any worth anything. I personally wouldn't trade my Mustang for anything. Its easy to mod and work on, and I think its the best looking mustang made. If was going to buy a new car I'd look at the WRX, yes, but I'd also look at a lot of other cars. If you drive the car and you like it better than your car get it, do what feels right.
Oh and don't get too caught up on acceleration. Its a good way to end up wrapped around a telephone pole.
And if you went to a WRX sight and asked the same question and linked them to this post they'd talk like the WRX was the best car ever made and that we are all full of crap. But we do the same thing. Car people are not very impartial.