[DING-DONG YO]

If your winrate is more at WPEX before the rakeback, I think you've answered your question as to where you should play.

As for difficulty, IMO, I think we all have different styles of playing that work well in different games. Your style might just play better to the games at WPEX. That in turn might lead you to the conclusion that the games there are "easier" when in fact they are just more suited to your style. The games at WPEX play differently than any other site which can be profitable for some but frustrating for others.

Or you're running hot.