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Creatine monohydrate is creatine with a water molecule. This is far and away the most widely studied form of creatine. There is a mountain of research to back up its effectiveness. The only downside to creatine monohydrate is that it does cause some bloating and water retention in some users.
Creatine anhydrous is creatine monohydrate with the water removed. It provides slightly more creatine than creatine monohydrate on a gram-per-gram basis, but it has never been shown to be more effective."
I'd just got with creatine monohydrate at a good price and 5g a day.