http://88hybrid.com/forum/viewtopic.php?t=7459
there is a post on a z31 board i made a few months back, we ended up getting into a mk3 vs z31 debate.. i dont think they faired very well.
I was into Z31's before supras, i test drove multiple ones, i actually found the mk3 supra by accident, was looking for 86-89 celicas and selected supras on autotrader. Anyway saw the NA had 200hp, lol the z31 had 165 so went and test drove it, That sealed it, bought a supra and never looked back.
The Z31 from a performance standpoint holds no competition. The Supra bashed it in every way possible even with it weighing in at 500lbs more. the NA supra was more of a match for the turbo z31. And there was no contest Turbo vs Turbo. The supra out handled, out braked, out performed the z31 on every road course. And you can tell why if youve ever gotten under each. Ive had a supra for 6+ years now, ive done everything from NA's, turbo 7m's, to turbo 1j's. I recently bought a 86 300zx to flip and OMFG im glad i went with the supra. The rear suspention is a joke. its a shock spring setup and pivots from a single crossmember going across. No box setup like we have, or unequal length control arms. They are known fro having horrible rear end sag. Not to mention that the rear differential is actually a large protion of the rear suspention strength. its actually used to hold it together unlike the supras which the rear subframe is the strenght and the diff is merely mounted inside it.
The drivetrains are again no contest in favor of the supra. The NA transmission gearbox is about the circumference of a grapefruit and are extremely weak and flawed. Ive had three that have destroyed the input bearings. The motor isnt even as strong as the 7M, id say the 7m is comparable tot he z31, and the 1j mk3 is comparable to the z32. The Turbo version is non-intercooled and the injectors are junk as are most Nissan injectors. The car is a PITA to work on, and has a very small performance aftermarket. My buddy has an upgraded turbo, aftermarket intake manifold, intercooled, with larger fuel rail, 550cc injectors, tuned on 13lbs. And when he raced my brothers bpu 7M on 14lbs(stock turbo, injectors, IC, motor, untuned) My brother just ran away from him. My buddy could barely beat an SRT4.
Now there are a few things that the z31 has on the supra. It does have a few more advanced options, like a dual tripo-meter, the turbo versions have a nice factory stereo with 11 band eq, not to mention that the car is lighter than the supra. But the interiors are FUGLY, definately loose in the aesthetics department.
Really between the z31 and the mk3 from my standpoint there is no comparision. Even comparing it to the z32 id still choose the mk3, as ive worked on those as well, and as its been said is a much harder decision. I can get into that if youd like as well, but you asked about the z31. Hope this was helpful