Normally I always go Running Back for my flex spot, and Cedric Benson is not a bad one. His matchup is against a stingy San fran run defense which could pose some problems. At the same time though, San Francisco has a terrible offense and I actually think this game will be tight, meaning Benson will be carrying the ball the whole game through (something that may be rare with the Bengals this year). In this sense, I think Benson is a solid start and I would play him over those receivers.
The receiver that could potentially merit is a start is decker, but even still I would be more confident going in with Benson.
Take it for what it's worth, I'm a running backs guy after all http://www.fantasyfootballcafe.com/forums/viewtopic.php?t=498295