They're fairly similar demographically - the main difference is that RM has a larger proportion of Asian students; QO has a larger proportion of white students (other groups are represented about the same at each school).
In terms of average SAT, RM's are 100+ points higher, and the % of students scoring well on AP/IB tests is also much higher at RM. The IB magnet program (nearly 1000 kids from all over the county apply, and a little over 10% get in) at RM skews those numbers, but I don't know how much.
I have a child at RM - not in the magnet program - and I'm happy with the academics. I think sometimes the perception is that kids in the magnet program get a great education and all of the other kids get thrown into dumbed-down classrooms with peer interactions straight out of Lord of the Flies. (okay, maybe not quite that bad

)That hasn't been our experience at all. My child is getting a rigorous education - in addition to the IB magnet, RM also has a robust AP program.
I don't have any direct experience with Quince Orchard, but my impression is that it's also a good school.