I played Counter-Strike for years with a trackball. Once I realized no one else did I started running around named 'Trackball Avenger'.

My personal take is that the index finger ones are easier to switch to and are a little easier to control, but I agree with Katharsis that the thumb controlled ones give you better button control. The only possible exception here would be to get one of the Kensington Expert Trackball Pro here but I can't seem to find a place that actually has them in stock and it's big $$$. I also think the larger the ball the better and easier to control. One thing with this kind is that the ball is so big that you can actually control it to varying degrees with all of your fingers, so if you slide your hand over to the left you can work it with you pinky and vice versa with your thumb.

It's the only one I've seen with enough buttons that I wouldn't be constantly moving back and forth from my 10 key to my mouse and back. If anyone else has seen an obtainable trackball with a sh*t ton of buttons please post. The Evolutionhas 6 but that's still not quite as many as I'd like

Kensington Pro 64214Evolution

Given this I've just purchased a fang game pad, hoping to reduce mouse use to looting alone, we'll see