I've been a Ford guy since my first car, an 88 5.0. I still own it 2 decades later. I was set on buying a '13 Boss 302 until I looked more into it. I've driven plenty of stangs, including the newer GT's but the dealership wouldn't let the Boss leave the showroom. Shame, they might have sold the car that day. However, I couldn't get over the interior. So much space and so poorly utilized.
The GT500 is better and if you're looking at the '13 you can get most of the amenities that you'll get in the M3 but when you sit down in the car it will be blatantly obvious that it's still, at its heart and soul, a Ford. Unrefined. Still a damn good deal for a 662 hp car though.
If the M3 bores you then maybe the GT500 is exactly what you need for a while. I can't comment on the Vettes, I'll never own a bowtie.