DL has EMH/MM on Saturdays and DCA has EMH on Sundays (baring some special event or unexpected change in the schedule).
Like Girimama33, I don't think EMH/MM has a long-term impact on crowds in the parks because park hopping is so very easy. That being said, I do think EMH/MM has an impact on crowds right at opening. The attractions that were open during EMH and/or MM will already have lines when the park opens for everyone. For some attractions, like RSR, TSMM, Peter Pan, and Space, these lines can be fairly lengthy. If you went to the park on a non-EHM/MM morning, the lines would be shorter because guests would not have already had access to the attraction for an hour. However, by mid-morning, I think it all balances out. So EMH/MM only really impacts the length of line a guest is likely to experience right at opening.
DL's EMH/MM only includes Fantasyland and Tomorrowland, so as long as you were heading towards Adventureland, Frontierland, New Orleans Square, or Critter Country, I think it won't make a big difference which day you are planning on going to DL. More of DCA is open for EMH (including Carsland and TSMM), so those attractions will already have lines at opening if you go on the EMH morning.
I generally do as Girimama33 suggest. Start in the park without EMH/MM and hop later in the day.
