P.S. The only real difference is the fee charged by the pool. Slush charges 2% versus Deepbit which charges 3%. So over the long run, you'll make more with slush.
Other differences include long polling support and invalid protection. Additionally, latency between your workers and the pool can have an effect. Simply comparing fees isn't the only factor to consider. If it was, everybody would drop Slush & Deepbit to join BTCMine, bitcoinpool, Eligius, or my pool.
That said, in my time using both pools, I found no noticeable difference even when running 3 gHash on each pool. The fee difference was roughly equivalent to the increased efficiency on Deepbit, so it was a negligible net difference.