Well either would be a good fast car if built right. Right in with the specs you listed I'd go with the mustang cause more then likely the 302 will walk all over the 305 T/A.
Make sure the T-tops are not going to leak like a sive all over the place if you go with that one. They will get water inside if you apply too much pressure around different areas of the weather stripping.
The mustang could of been ragged on pretty good beings a 302 was dropped in place of a 4 cyclinder.... but pay insurance on a 4 cyclinder vs a v8 isn't too bad either.
If the TA is fuel injected not throttle body you can get yourself a nice 350 set in there with that and have a nice running car. It's all up to personal preference.