iirc, I used SW Emerald flat on walls with SW Emerald satin on the trim. I've had to clean up a minor splash or two, but it was nothing that required scrubbing or harsh cleaning products so it cleaned up fine. If you expect the need to clean your walls regularly or rigorously then you might want to go with something like satin.

Susan C, if you're still there... I just found a bit of my leftover Emerald paint and see the label is marked "matte", not "flat". They may be using the terms interchangeably, so ask to see samples of their paint finishes -- that's what helped me decide.