if you prefer a simpler camera - for your own shooting style and aesthetics - any of the M6 variations will do. as isaac said, what matters is the camera's individual condition rather than it's vintage. any of the M6s are old and subject to the usual mechanical and electrical deterioration with time.
if you would like auto exposure control and other less frequently used features like timed 30 sec exposures for night shooting, an M7 is for you. practically less subject to age-related deterioration because it's newer to begin with, they can need repair too, depending on pattern of use (or disuse).
preferences are personal and can amount to hair-splitting debates. get one, shoot it, and you'll find out what you really prefer. worst case is that you sell it and buy the one that you've learned from experience will suit you better. which isn't bad at all.