OP - You have your answer.
If you spend more than $25.00 on oil for your 30K+ Mustang, you're a fool. How fucking dare you put good oil in your car, what the hell are you thinking?
Seriously, though. Buying $70-$100 in oil should not be a determining factor when looking at the S197/S550 platform 5.0's. It's money well spent on a badass car.
Ignore the haters as people here are extremely sarcastic and love to be keyboard warriors and internet badasses. Buy what you feel is the way to go. Personally, I love the 11-14GT more so than the 15+ GT's.
You can make great power with either one. I'm @ 443 RWHP / 388 RWTQ with full-bolt-ons on 93 Octane with some upper RPM Knock. I'm sure I'll be well over 450 RWHP with better gas. Both cars are great, but, I will admit that the interior on the new S550 is amazing, although, the 11-14 interiors are not bad, especially if you have Navigation (thankfully I do)
Good luck in your decision. Hope to see more of you on S197forum. It is a great place, just thicken your skin a bit and ignore those that want to bring you down bro.