I just traded in my 09' WRX for a 13' Mustang GT and I thought I'd talk a little about how they stack up against each other.
First off you can't really compare these cars but I have owned them both so why not?
I loved my WRX for the 3 years I had it, it is what it is and doesn't pretend to be anything else.
I originally ordered a 12' wrx, but during the 9week wait I thought to myself, do I really want to trade in for the same exact car with a body kit? No
I test drove a 13' 5.0 and fell in love. It has a completely different presence on the road. It feels planted and composed, it's suprisingly smooth compared to the WRX. Im still breaking it in so I haven't gotten on the throttle hard yet. But I can say there is plenty of power down low compared to the WRX in its stock form, where it was dead below 3500rpm and insane from 4500 to redline.
I bought both cars in Base trim. Interior wise the GT is top notch. The WRX was seriously lacking in this department. The GT has an all black interior including the headliner as apposed to the 2tone cream and black in the WRX. The GT has soft to the touch plastics on the dash so it doesn't creak and squeek in the morning. The GT has steering wheel controls standard ( I didn't know what I've been missing) which are awesome. The keyless entry system is one piece with the key, not a 2piece as in the WRX. The stereo is much better than th WRX which goes without saying. Hid's are standard also, which is nice. MPG wise, the 2 cars are rated the same which is crazy since I now have 2x the cylinders. I'm getting 20 right now ( 90%city) which is great considering I'm in the break in period and I'm giving it more gas than I usually would to vary speeds and rpms.
Things I like more about the WRX than the GT. I like the WRX gauge cluster much more, it's so easy to read and the white on red looks awesome. I'm not gonna lie, that's about it. I live in San Jose, Ca where we don't see any snow and hardly any rain, so AWD isn't necessary for me at this time. The WRX has a bigger trunk opening, but I think the GT has more useable space, especially with the trunk lid arms on the rex that prevent you from putting anything below them.
All that being said, I would without a doubt buy another Subaru, although it was lacking in many areas it's still a great bang for the buck and you can't beat the reliability. The GT's reliabilty is TBD but it seems to be solidly built.