I like the styling on the '12 GT500. As for the '13, the front end looks a bit more aggressive, almost like it's trying too hard, but I do like the quad exhaust in the back. Honestly, I think I'd add the Ford Racing 750hp supercharger upgrade before I would trade my car in for the '13.
I like the 2013 looks...I was originally a mustang guy (1966 GT as my first car, a $40k restoration...and a 1988 GT with T-tops and the full FMS catalog on it plus some Lincoln options due to being the regional service manager's personal car when I bought it in 1990).
The latest (esp the rear end) have been atrocious. Not to mention Ford allowing the cheap seat crowd to buy the premium body packages.
You do realize that 750HP kit is like 30% of even a 2011 model once installed.
The premium Mustang crowd updates every new style to stay in front of the pack.
I was shopping the Boss 302 vs the 2006 M3 I ended up with. They were within $50 per month payment wise (I went with an ultra low mileage M3). Still every test drive in the Boss I passed probably 12+ nearly identical cars.
Personally I want a unique car rather than the fastest out there. I could easily build something 'fast', but it's not that fun always. The 1988 GT I owned, I actually downgraded some parts to make it more enjoyable day to day.
My 240SX I recently sold was an OK commuter when I was less than 4 miles from my office, traveling 30+ miles each way in my next gig was too much.
I loved that car.
I will agree that sometimes paying much more than a car is worth is the right choice in rare models. In your case I'd probably source a Boss or GT500 or a 3rd party's version rather than dropping so much on straight line power.
Don't let a GT out power you man. 
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