Personally, I like to do MMRR first just because you're right there walking past it on your way in and most are heading towards GE now to do RotR. And if you're lined up before ETPE opening, you should get through MMRR quick enough to get a low wait still, theoretically speaking anyway, for mfsr.
I would do it the opposite. You need to get to the park at least 30 minutes before EE as the crowds will be (at least they were our week). They let us in the park about 8:15 and walked back to get in line for RoTR. The line started to move at 8:30. We waited about 20 minutes. When we got off the ride, about 9:15, the line looped out to Muppets and all the way back.
walked over to MMRR and waited about 20 minutes.
Had we done MMRR, I suspect the RoTR wait would have been 45-60 minutes.
If the line is long for RoTR when you get there, it may be worth doing MMRR first and go back to RoTR later in the day. At least the days we were there, that line got down to 30-45 minutes mid afternoon.