Bigger pools are not necessarily better or worse. What they are is more consistent. The larger the pool, the more regularly a block will be found, and thus the more regular of payments. More important than this regularity is the pools fee - my primary pool is
www.btcguild.com, which operates purely on member donations and is in the top 3 largest pools. I used to mine with deepbit (the largest pool), but stopped after I decided the fees were too high.