I agree with dragnut, but would go a bit further and recommend you make your decision based on actually shooting several candidate guns. The one you shoot best may not be the one that feels good when you handle it in the store.
I'd further recommend that you make sure you understand how to properly stand and grip the gun so you can make a valid comparison.
Finally, take into consideration the placement of the controls and how easily you're able to perform the manipulations required to load, operate, and clear the gun.
Realize that no matter how thoroughly and methodically you do your research and make your selection it's almost certain that you'll eventually find something you like better. But, your first gun will have paid for itself and done its job by serving as a vehicle for learning.