From what I understand, 51%'ing a network does no harm unless the pool operator has bad intentions.
Why risk it though? What does ghash.io give you that BTCGuild or eligius doesnt?
BTCGuild = 3% Fee (Thats ALOT when it comes to bigger BTC amounts)
Eligius = due to some problems i seem to get there around 2% less than ghash.io
So it DOES matter where you mine.
Nevertheless it should be obv that its time to mine somewhere else regardless the performance.