Being a fan of "JM" stamped Marlin, I would find it hard to part with the Marlin. With that being said, it sounds like a fair trade. The rifle you will use the most will probably be the one to stick with or trade for. The two are very different creatures from one another.
On a second note, If its a buddy your thinking of trading with, I would shoot the DPMS and check for accuracy.
How difficult is it to find a copy of the Marlin you have?
How difficult is it to find a copy of the LR-308 you're considering?
Then, what's current market value for each?