I'd always go with the RB.
I think Roy Williams is going to be a stud (I understand that many think he's already there). But remember, he plays in DET, whose shell of a running game is starting to sputter. Without that protection, I see Williams drawing more double teams and his production taking a dip.
Also, rookie WR's almost always struggle a little bit (the exceptions that come to mind are Randy Moss and Anquan Boldin). He's certainly a solid #2 WR, but people in my league are ready to crissen him a #1 WR, and I think that might be a bit premature.