I would have to say go with the supra, though it is a little heavier, and I havent a clue as to where Redbeauty is getting his information the 7MGTE is a VERY realiable block.
Stock for stock, the Supra has 30 more horse then the Z. Its is an inliner, which I think is a much better choice, dual overhead cam. Also, comes factory with an LSD, no search for one or having to pay outrageous price for a used one.
They handle very well, are adjustable as far as alignments go at almost ever point front and back.
I have owned both and would say the Supra is the better choice. You will find that a lot of the Z31 crowd carries a bit of disdain towards the MKIII supra.
Pesonally I think its a great car and you cant lose with them. I wouldnt look for on on ebay though as they get overpriced there, as with many things.
Good luck in your choice and search