You will definitely get a much smoother sound out of the Dyn's IMO, although Rotel and Dynaudio can be a bit cool for some tastes. Audition both if you can. If a shop carries dynaudio with rotel than more than likely they will have B&W there too. Rotel pairs fairly well with B&W.
It's been a while since I heard Mission speakers. My memory of the m52s is of a big sound. I think that the 42 would be a sideways step. Yes, they aren't as bright as the Missions, but I think you may miss a lot of the scale the Missions bring to the mix. I suggest an upgrade would be the 52, although that costs quite a lot more than the 42.
In pure compatibility terms the Rotel is more than capable of driving both 42 and 52. Typical Rotel really, a lot of bang for the bucks...