Apparently there's a second, completed, ROM that was never released. It contains features the leaked ROM doesn't, at least according to the folks who worked on SF-Zero, who played it as research.
Star Fox 2 has some interesting additions: it seems more fast paced than the first one, the ship can transform in certain levels (in Star Fox 64 a different vehicle would be deployed to the level rather than transform), there's Star Wolf and some new crew members that make some appearances in Star Fox 64.