I'd say it depends mostly on the situations in which you'll be using the lens. The MRC coating does two things - 1) reduces light loss through the filter (the multi- part) and 2) makes the filter easier to clean (the resistant part). #1 is pretty irrelevant with a 10 stop ND filter, where the whole point is to cut down the light anyway, so a little more lost due reflection is not bad. If you will be shooting breaking surf, etc., the MRC coating would be a definite advantage.
Personally, I have the single coated 77mm 10-stop, and the uncoated 82mm 10-stop, both bought before the MRC versions were available (and the 82mm is the Schneider Optics version, B+W's parent company).