I like Josh Phelps, I even picked him up on a team but if Marrero is available I would go that route...he is the third highest rated C in 4x4 ML . He has been there consistently all year while Phelps has not proven anything...If you are in a keeper league though you may want ot invest a claim on Phelps.
True, Phelps hasn't played enough games to be considered a safe option yet, but since you need the help in average, he's tempting. If you feel you'll reach first place anyway or if you're trying to defend second, go with Marrero; if you're desperately looking for options to push you into first, Phelps' upside starts looking pretty good.