The Sega Master System version of the game is actually a pretty good version of the game when compared to the Mega Drive version. I've only played the Mega Drive version myself, but do remember it being ridiculously easy in difficulty. But hey, I don't remember there being a much better experience than a co-op game, robots and humans working together, battling out against aliens.
Reminds me of the new Prometheus movie! Alien Storm: the prequel to Prometheus: the prequel to Alien.
Gameplay Footage: Alien Storm on YouTube