Rey going to Raw would be a pretty bad idea. Rey is 1 of the company's biggest draws, esp. among kids & the Hispanic demographic, so moving him to Raw (which already has Cena, who already has the kids locked up), seems like a redundant idea. Not to mention he'd be slotted no higher than 4th in the pecking order, behind Cena/HHH/HBK. What's the point? You'd just be devaluing 1 of your top draws, and for what?
Leave him on SD. Who else, outside maybe Taker, is going to draw for the brand? Move Rey off the show, and they'd just be removing their biggest demographic that watches it.