Alien Shooter is a game that will feel eerily familiar if you have played Zombie Shooter already, and that's because they're essentially the same game. There are however some minor differences on account of Zombie Shooter being the later of the two.
In this game you can't upgrade your weapons, the enemies are actually less innovative than in Zombie Shooter if you can believe that and the graphics are somewhat more dated.
Alien Shooter also suffers heavily from the fact that it uses smaller maps than Zombie Shooter and compensates by introducing infinite enemy spawners which is just a horrible idea.
In the end, it's like a slightly crappier version of Zombie Shooter, everything is just a little worse and Zombie Shooter wasn't a very good game to begin with. If someone holds a gun to your head, pick Zombie Shooter over Alien Shooter but if you still maintain your free will, don't bother unless you got it through the Indie Gala in which case you can get an hour or so of fun out of it.