I don't know much about the differences - I just knew there were different types! I think H have a faster data rate than M, and H are meant to be better for video (or more to the point you may have issues doing video with M). I don't know what compatibility issues there may be, but this sounds like it could be the significant difference. If you find the M is too slow it might be worth trying the M+ (I think this is the same as M, but faster).