It all depends on what his point of view is. If he's content with staying where he is because of the schedule, he should stay. If he wants to make a bigger name for himself, he should move to the WWE, as they're currently struggling with a lack of available talent. They have two struggling midcard divisions, two dead tag team divisions, and at times the main event becomes so repetitive that its a bore.
If Christian did in fact come back to the WWE, I'd place him on Raw's main event scene, pretty much as a replacement for Mysterio, who I feel is best suited for the "upper midcard". That would allow Christian to have his feuds with Jericho, Orton, Cena, Punk, HBK, etc, while allowing Mysterio to start putting over talent like Burchill, Cade, and Regal.