I would recommend the Cl. I was in a camera store last week and we were discussing a Cl. There are no spare parts available for them. Repairs/parts are taken from unrepairable Cls and they are expensive and the turn around time is long. The Canonet is a good beginer and if it goes bad you are not out a lot of money. Another camera is the Yashica GSN; it is less expensive. It is larger but the lens sharper (IMHO).
There are a number of places to have a CL serviced properly, plus when it is serviced, it has some value.
Is the build quality as good as an M6? No. But neither is the M5.
I have a dozen or so fixed lens RF's which are not worth the CLA to get the shutters back to reliability. OTOH, I have a Petri and Ricoh RF which both are working well (no meters).
Prices? I know one is getting an overhaul, with a replacement mirror, RF rebuild, full shutter clean, and meter adjustment for the alkaline battery, $80 dealer cost, hardly expensive. The lenses on them are bringing good coin now for whatever reason. I was really surprised at the price, especially since mine will cost me more. It may also depend on the age, I have been told the early ones had more problems with the meters.
Turn around time, couple of weeks.
If the price going in is too high and you still want an M mount, I would say find a good user M6, if not, you are not likely to lose much to give it a try.
Regards, John