oh man the Megadrive version is insane. time limit, Pouncing aliens and the levels are ridiculously complex. and the Sound effects, damn.
the SNES is more slow and you have more varied objectives and enemies, but it becomes tedious when the spawn points start to show and how it takes you a while to get from place to place because Ripley can't run for shit.
hopefully this game will be everything these two and Contra can offer and more.