I'm a HUGE Mustang freak, but the Supra is an awesome machine even in stock form. No dissin from me, BUT, overall, you can make that Mustang Quicker, Faster, and handle better than the Supra for far less. Now, I highly doubt you will be able to make 9 second power on a 5.0L, will definately have to bump up to the 351 Windsor or Cleveland(windsor would be the wiser choice, and stroke it for reliability. Personally, I happen to like the fox platform. They are cheap light and there's a myriad of parts available for them. If you break something, it's so easy to get new parts. Good luck finding parts readily available at a moments notice on the Supra.
As for the Supra, I've read many times how the stock bottom end is capable of supporting upwards of 900HP. Nowhere near that on the Stang. Hell you'll need quite a bit of support to run 500 on the stock block. If you go Supra, I suggest no body kit. The stock lines are downright sexy, yet subtle and IMO, should be left alone. A nice set of wheels and you're good to go.
Either way you go, you'll have an awesome machine.
And enough of the import bashing. I used to be that way, but think about this........
Why are the BIG 3 taking their work out of the country to make money, when the import manufacturers are building plants all over the country here and they don't have a problem do they. Hmm...........