That's a tough one. I have both guys on my roster, in a must win (down by a good bit) night in round one of our playoffs. I have Halladay, Marcum, Hudson, and Nolasco going tonight, and if they can't get it done, I'm done. I've decided to start Halladay over Marcum and sit Marcum, because if I start both and one wins while one loses (especially if the losing one has a bad game), the points for winning in my league are equal to the points for losing, so they cancel one another out. So I'd be betting on the loser of the 2 scoring more than 20 points. Meh.
In your situation, I would say sit Marcum. With your roster, you should have no problem finding 100 decent innings from that group without having to dip into Marcum tonight. Just my 2 cents.
U: Cuddyer, Aoki
B: Ackley, Morneau, Simmons, Freese
SP: Bumgarner, Dickey, Cueto, Kennedy, Sanchez, Samardzija, Minor, Vogelsong
RP: Johnson, Wilhelmsen, Perkins