DM's are usually faster paced and more of a military based-rather than an economy based RM game. Having said that though, many of the "expert" DM'ers know the importance of an economy and typically have villager highs close to or over 100.
Most often, DM'ers don't worry about resource management for the first several minutes in the game. That is a large role of RM and it takes a lot of thought and skill managing a small economy.
Also there is less of a talent gap between players because there are less decisions to make in the game. In DM's you start off in Iron, and usually start attacking with Super Units of Armored Eles, Balista Towers, Heavy Cats, and Helepolis. Building patterns are important, but not so much as RM's.
In RM's you must get good economies in Stone, defend and attack in Tool and Bronze while still growing your economies. Super units like Heavy Cats and Armored Eles never really play a role in determining the outcome of an RM game.
Personally I think RM's are a lot more fun because they require a lot more skill. RM'ers typically make great DM'ers when they make that transition. The same can't be said about a DM'er going into the RM world.