This question still tears at me as I ponder my next Mopar project.

I've been lucky to own 4 Trans Am cars (2 T/A & 2 AAR) and granted they were race versions built for the street, it's still 40+ year old technology. It'd cost you a bit as mentioned in other posts to upgrade suspension, steering, etc.
Now that being said, I just sold a 2010 Challenger Mopar 10' edition last year and man, that car was fun fun fun!!
Newer car, a/c, and had the track stuff built in from Ma Mopar, front/rear strut tower braces, cold air intake etc.
I took it up to Road America last year for Vintage weekend and got to take it on the track while my friend snapped pix and took video. WHAT A BLAST!!! Handled like a dream and stuck to the track like glue!

Now I know you asked specifically about the Mustang, and if you can find the model you want, I'd say go for it and experience the difference.
Those Boss 302, Leguna Seca, and especially the Shelby GT500's are just beasts. If you've owned mostly old cars, try the new Stang, Challenger, etc and I think you'll see you can have it both; a fast car and the creature comforts that come with it. My M10 had an added nav system, and on a 90+ degree day, I had no problem going for afternoon cruise.
you get in a 70 something car, and you aren't gonna want to leave the driveway with no A/C.
Its a tough choice, but I'm sure you'll make the right one.