What size is the Marin? IMHO riding the wrong size bike is a recipe for not enjoying the ride. A hybrid won't be too fun to race even if you change out some of the parts.
It looks like you have quite a few spacers on the monster cross and taking some of those out will change the feel and which muscles you are using. Sacrificing a little bit of weight on the bike for a more comfortable race type fit would be the way I would go.