Zombie Shooter 2 is pretty much just a slightly upgraded version of the original. However whereas with the two original Zombie and Alien Shooter games Zombie was the slightly better game with the sequels Alien Shooter is the superior speciment, for what little that means.
They're still pretty much the same game. For the sequel a small RPG feature was added and the maps opened up a bit more. Unfortunately it's still a pretty crappy game, not really worth your money.
Thanks to the Indie Gala however many people will end up owning this and if you do you can get quite a few more hours of out this because it's done away with the atrocious short campaigns of the original. With a bit of exploration there is 5-6 hours in Zombie Shooter 2 and while it can be good for a bit of fun the game is just such a low budget money grab that you will feel a little dirty after playing.
Best selling point of Zombie Shooter 2 is the dialog and voice-acting which is so bad it actually is hilarious.