ECA has caffeine in it...caffeine works as a diuretic, so it triggers water loss. Creatine does a few different things, one of which is cell hydration. So, the caffeine does work against that benefit of creatine. Creatine does two other really valuable things though: it has an incredible strength-building effect which can allow you to train heavier, AND its a great antioxidant. So IMO if you really like you ECA for energy, and don't mind that it detracts from the muscle fullness you get with creatine, then go for it.