This one is just too much... okay, who made up a fake screen name?

Edit: I just realized that I sound like a complete *******, so just for the sake of helpfulness, here we go:

You'd have to ride the bikes to know which one is the "best" bike because for entry level track bikes, it will really be subjective (and it will also depend what components you put on it). However, if you're REALLY desperate for advice from a stranger online, this thread is a good place to start

http://www.bikeforums.net/showthread...t=budget+fixed

As for the cog question, it will affect both your gearing, and how many skid patches you will have on your rear wheel. For more details about those, you can use the search function, or just wait a day and a thread will eventually pop up.