Statistically, it doesn't matter what pool you join, in the long run, you will make the same regardless of what pool you join.  The only thing that can impact that is the pool's fee structure.

Big pools give you smaller but more frequent payouts, and this plays to a lot of miners' psyche.
Small pools will give you larger but less frequent payouts. (this is called variance).

So as far as how to pick a pool, looking at it from a profitability's perspective, look at their fee structure.  Otherwise, pick one with a good community (or not).

Personally, I would go with the smaller/mid-sized pools, they have a better more homely community and it helps spread out the hash rate.