Hmm... I'm in a points league, so I never start my pitchers against each other. I either pick one, or bench both.
Brewers and Reds are huge hitting teams - especially at home for Brewers, while Reds have proven they can hit on the road. In a points league that's a lose-lose situation and I would bench both.
On a side note, I don't consider Cueto an ace, so I wouldn't have have started him in this one anyway. Marcum's got a better chance at getting a W because of Brewers' home record. If I was going to start either I would start Marcum and hope for a W.