I dabble on both, and I find (as I should find), that slush's pays more on average if you give it long enough to deal with the bad luck vs the good luck, and the BTC Guild's PPS is just a nice solid method with easy payouts.  It's just these really, really bad days that hurt things.  Over the last 2 days, with this huge spat of bad luck, basically have lost a full days worth of mining compared to a PPS.  But let it ride, and eventually it should even out.

So, it comes down to personal preference.  PPS for a zero variance (excepting of course difficulty changes) reward, or a score system like slush, which pays based on luck.