I tested BTC Guild and Slush's pool, and I'm making nearly twice as much on Slush's pool. Go figure. 
How long was your test?
I calculated my estimated BTC per day based on my total hashing power and the current difficulty, and I'm getting very, very close to that on BTC Guild.
Indeed. If you try a new pool, you need to leave your miners on it for at least 72 hours straight, and measure your earnings for that time period.