If you're considering a used 2016 ATSv, you should be comparing it to a new 2016 SS (or barely used SS).
The price difference is about $15k.
As others have mentioned, there's a significant size difference as well.
Performance wise, ATSv has a significant advantage.
Total cost (depreciation), if you buy an SS new for $38k, the cost of ownership will also likely be less than a $54k ATSv. Just look at the used CTSv prices to get an idea, and that car is arguably more desirable in the used performance world with a larger aftermarket and tried and true drivetrain. The super fast depreciation already on a low volume brand new model should be worrisome for long term prices.
Summary- ATSv is nicer, smaller, and faster, but it's going to cost you a LOT more than the $8k difference you're thinking. Just gotta decide if it's worth it to YOU.
Since you can afford a GTR as well, I don't think the price is the issue. So what are you really looking for in a car? If the answer is:
RWD, v8, comfortable for 4 large adults, nice ride/handling/brakes, fun to drive daily driver and you don't care about fuel economy.... the answer is SS.
But there are certainly better performers out there.... it'll just cost you.
There's a good "super sedan" comparo out there worth reading. It had the M5, E63, Panamera Turbo, S7 and XFR in it. The interesting thing was that the SS out handled and out braked EVERY car on the list except the $180k Porsche.
However, even the slowest car on that list was significantly faster in a straight line than the SS. Good thing the cheapest one of the group costs $40k more!
2015 SS. Heron white, auto, roof, spare
1969 Ford Mustang Mach 1, s-code 390