If I really did some digging I could probably find an 05-07 mustang and even possibly the GT. My question is, is the new mustang (referring to the 05-07 models) noticeably better (in a straight line) than an older model? Old v6 vs new v6, old v6 vs new GT, old GT vs new GT, old GT vs new v6. If I can spend less money on an 04 v6, will it ever be as good?
Most important thing is to aim for a body style you love. The S197 is different inside and out from the SN95. For my taste, I preferred the S197, looks more late 60s early 70s, where the SN95 has its own look. The interior, to me again, is also much nicer. Now if you look at the two and feel the SN95 is better, then that is what you should go for.
The S197 has better balance in weight distribution for handling, a little less nose heavy. The SN95 has a lighter rearend, so easier to smoke the tires, drift the car, or spin out.
The S197 has improved suspension over the SN95, and this increase becomes larger from 2010-2014.
The GT has a V8 and the base is a V6. A V8 has more power and sounds different than a V6. Which ever you prefer, go for that.
I have a friend whose 2 sons each have V6 SN95s. They love their cars and have done plenty of mods to them. He and I each have 2010 GTs and love modding them. Our cars have the 4.6L 3V - it was 2011 and up that got the 5.0L 4V. Lots of folks on here prefer the 5.0 as it comes with 100 more horses. But I have beaten a 5.0 on the quarter mile, the 4.6 is a great engine with few problems, and it has some advantages in modding and costs. Shelby offered the Terilingua Mustang - a V6 set up to out track race V8s (S197 platform). So you see, any year Mustang could be the right one for you. But you have to drop buyers remorse and focus on what is good about yours or you will always have regrets and not enjoy what you have.