I think I'd start all three RB's and then I'd go with BMarsh over AJ, the Texans have been playing really well against the pass. Over the past 6 games they've only allowed 2 WR GROUPS to go over 100 yards. Marshall plays better when he has top corners on him, see Revis/Cromartie, and Moore looks his way often.
As for the Defenses, I like the steelers, the Browns don't have too many weapons and they Steelers have been playing better as of late. The niners is also a solid option, and if you chose to go that route I dont think I would argue with you about it. I just think the Cardinals have more weapons then the Browns, I feel the defenses are about equal.