The big ones attract more attention.

There was a correlation between school holidays starting and increased attacks. Those script kiddies get pissed off when they try to activate a botnet and get their Deepbit account automagicly got suspended once it gets over 150 different IP's (which indicates suspicious activity). Slush is a bit more forgiving on that part.

As for choosing a pool, it's frustrating to be on a small team (like Eligius.st) as it takes a few days to see result (and get a possible payout). You do get the same payout, averaged over time, but once a while the pool hits an orphan or an invalid block, and that means you've been crunching away for days and don't get paid. Deepbit does pay for those.

I have deepbit setup to pay once a day, which means I have a steadier stream of income to spend.