Just looking at the pictures of the respective seats of these two cars reminds me of one of the reasons I prefer the GTO. The Pony's retro seats feel pretty nice till you sit in the GTO & you're caressed by its superior thrones, front & rear.
When you come down on the go pedal from a crawl, you'll notice the slight lack of push in the seat of your pants from the GT, compared to the GTO.
The slightly "dry" sounds emanating from the engine compartment of the GT under acceleration compared to the deep rumble in the cabin of the GTO.
If a car's function while really "driving" is more important to you than the main street parade scene, then driving these cars in quick succession (I did) on some curvy tarmac will reveal other advantages of the Pontiac:
If the pavement is anything but perfectly smooth, you'll feel the rearend begin to dance out with the GT (what's with the live axle, fercrissakes??)
The overall superior dynamic composure of the GTO becomes evident in quick transitions over undulating pavement - the GTO feels like its on rails as the Mustang requires noticeably more attention to the wheel.
Contrary to some reports, I have found my m6 to be smooth & sure - as though it were immersed in thick syrup compared to the 'Stang's somewhat more notchy shifter. Neither were bad, the GTO's just more liquid.
Though I like the styling directions of both cars, in a year or two when the roads are filled with V-6 "Secretarialstangs", the retro-cool is gonna fade (ala PT Cruiser, but even worse).
Last, but not least, we all know you're not gonna pay sticker for the GTO right now, but they sure weren't dealing down on the short-supply GT right now.
After my personal evaluation of these two cars side by side, alas, I rushed back to my local Pontiac dealer to snag one of the last two blue M6's available in CA!! There's no doubt in my mind on which one "won" my comparison test!