While this is not going to a long post, we mined with Antminer S1s to BTCGuild for about a month and received around .0215BTC per day per Antminer. With Cex.io, we are generating about 0.026BTC per day. While it is not that much of an increase, the multiple payout addresses and easy in-site trading with CEX.IO has to put it above BTCGuild in our opinion!
How did you account for the change in difficulty?
I'm assuming they had 2 antminers. Of course, their 1 month period, if it was done as of their post or within a day of it includes the worst luck BTC Guild has had since it started tracking luck beyond difficulty boundaries 7 months ago, making it the worst 1-month period in BTC Guild's recorded history for luck.