they are two totally different helmets and it's pretty rare that the both helmets fit the same person. The WP is super deep, and while oval, is a little on roundish side of oval. The GR8 is definitely a more shallow helmet and long oval. I am lucky that I can fit in both, and while I currently have a Wellington Pro, I have had GR8s in the past. Go into your local tack store & try them on.
btw, I just have to say that I am flying in an airplane right now with WiFi on board and it is the coolest thing ever!