You have to think of the M5 as a camera that had to live through its marketing failure for photographers to recognize its greatness. Or put another way, it was the camera that Leica users were too stupid to know they wanted. I have both and love both. I had the M6 from about 1994 . . . bought it lightly used and loved everything about it having used two M3's for about two years before that. But if you forget for a moment that the M5 was sandwiched in between the M4 and the M6, and just hold the thing in your hand, look through the viewfinder and wind on the advance lever . . .man, there are just so many refinements in that camera. It is like loving a car from the 60's with fins and a carburetor, and an engine that purrs like a panther even though you know your modern Civic gets better gas mileage and is, ultimately, more reliable.
Now in the poll above, and despite my affection for the M5, I chose the M6. Why? Smaller profile, Rapidwinder and Motor-M compatibility etc. And, as they say, it ain't the years, its the mileage. All things being equal, I would choose the newer machine to take me reliably down the road. But I will never sell that M5 . . . love it warts and all.