Hello
I have played counter strike source professionally(2002). (No questions about that please) I have basically tried all mouse out there, and I am a mouse freak, so I have wasted a lot of money on tons of mice, luckily I have been able to sell them forward.
The first thing you should think about is the mouse shape. Most people like ergonomic mice. They are made for right or left hand, and they are really comfortable. Personally, I think those mice are bad for your hand. I like the hybrid ambidextrous mice. (made for left and right hand) The ambidextrous mice are symmetrical, they are the same on both sides. (I would like to mention that a lot of pro games as well like both types. There is no mouse type that is better than other mice, speaking of shape that is)
After you have found out what kind of shape, you should try to get a feel of your DPI preference. Some people are way too hung up on buying mice with high DPI. The truth is, that the DPI is way too high for most monitors to even discover. The movements with high DPI mice, lets say 6000DPI. The mouse has higher resolution than the monitor, which makes it a waste of money...
I just want to point out that there are still some, or a lot of pro games that still use the good old microsoft intellimouse. It has 400DPI (VERY LOW), and they still play better worse or the same as other pros.
The thing is, you need to find a mouse that fits your feel and preference.
I am using the Steelseries XAI mouse. It is a bit overpriced. But it is ambidextrous, and it has a better sensor than the microsoft intellimouse 1.1. I have my DPI at about 2000.
One more thing. Choosing a mouse, almost most important is to save money. There are 10000 shops that sell the mice, and the price is allways different. Find the cheapest one, and hopefully you have planned your buy for a nice mouse you will be happy with.