maybe it doesn't really matter to ya, but that model supra is heavy as hell (over 3k lbs iirc). And yes, although 7m's are infamous for headgasket issues, I'm pretty sure a prepped/built motor would have covered the whole torqueing the head down properly thing anyway.
Personally, I'd go for the S14. It's a lighter platform, and a T70 doesn't necessarily mean uber lag. But you failed to mention other supporting mods so I guess it's safe to assume that atm. It's definitely got potential and OFF HAND, I'd probably say it's cheaper to mod.