It's up to you on selecting the car you want to fully built.
A few things to consider is how supportive are the aftermarket industry to each car (which one has more parts supports out there), what issues does each one have (I don't know what issues Z31 has but the MA70 does have the issue of BHG on the 7M engines, which is not a problem since you plan on doing a full build so you can solve this issue with a metal head gasket on a rebuilt 7M or swap to a 1JZ or 2JZ), and what do you plan to do with the car (show, race, daily driver, a little of each). The MA70 is a heavy car at over 3000lbs. Don't know how the Z31 is compared to that.
Is there a reason why you are not considering a Z32? Do you prefer the Z31 body style instead a Z32? The S13 and S14 are also good cars and has plenty of aftermarket support, but a lot of people has one (cookie-cutter). If you don't mind having a car that many people also owns, these 240SX cars are nice too.
If you do decide on going MA70, this forum is a very valuable resource with members and venders to provide you advice and products.