I've got 5 ASICs and I'm presently splitting them between Slush(1) and BTCGuild(4).  I know that all mining should, statistically, produce the same result over the "long term" regardless of pool or even solo.  But I'm beginning to think that "long term" is a relative concept, and one that is becoming shorter and shorter.  The new "long term" may be a week in ASIC years.  In the last 7 days, the 1 miner at Slush has earned 26% more than ¼ of the 4 miners at BTCGuild.