there honestly isnt a lot of difference, and it is really down to personal preference and feel. I suggest you get one where you can adjust the DPI with the click of a button, its usefull for getting fine accuracy in FPS and then switching back to fast movement.
+1 for the Deathadder, it's a solid choice for a Razer mouse. I use an Imperator right now and it's got all the functions I need.
Just a note though for Razer mice - I've used a Copperhead, Deathadder and now the Imperator and for all of them I've found that the build quality isn't too great. The Copperhead buttons stopped working and had a problem where I had to replug the mouse every hour or so. The Deathadder's left click also stopped working and now the Imperator doesn't light up/engage on startup until I give it a quick smack.