I have an M6ttl and the M7.
M6
Basically a M6ttl without TTL flash metering and a smaller shutter speed dial that rotates in the opposite (traditional) direction (IMO no big deal). The finder will occasionally flare, but can be upgraded for little money.
Great camera, 20 years in production, what more can you say? Get the viewfinder / RF flare fixed and you have a winner.
M6ttl
Plus:
- The only mechanical M with full TTL flash metering. This is a pretty big deal if you use a flash.
- Slightly more sensitive meter (vs M6), but I have never heard of anyone noticing a difference.
- Reliable as a brick.
- Only the meter needs batteries. This xmas I was shooting for hours in -20C weather.
The batteries for the meter died, but the camera just kept on going. These are the same type batteries that power the M7. No batteries = almost dead M7 camera.
- Relatively cheap in the used market
- Will probably outlive the owner.
Minus:
- The finder will flare under certain circumstances (bright light at 1:00 o'clock). The .85 versions are the worst. You can have this fixed for $150-200 by Leica or a reputable repair shop. But regardless people did make pictures with these for 20 years...
- No on/off switch. You need to rotate the shutter speed dial to 'off' to prevent the battery from accidentally being drained.
- The black anodized finish can be thin and look like steelwool with age...
- Very high build quality, but not quite as polished as an MP.
M7
Plus:
- AE. Sometimes this a god sent for snapshots taken on the run. But I mostly use mine in manual mode.
- Quieter shutter (compared to mechanical M)
- All but the very earliest M7 bodies have the 'flare fix' to the RF unit / viewfinder and coated finder windows. The M7 also uses the improved RF unit from the MP, resulting in slightly higher contrast framelines and focus patch.
- Improved fit and finish and as reliable as a brick.
- 50 - 70 rolls per battery set. Keep a spare set taped to the strap.
- DX reader
- Exposure compensation dial
- TTL flash metering. Aside from the M6ttl this is the only analog M with this feature.
Minus
- Shutter depends on batteries, except for 1/60th and 1/125th. Tape a set of spares to the strap.
- Early models have a tiny gap around the eyepiece, which may cause a minimal amount of dust to enter the viewfinder. It's a trivial problem and can be ignored. You don't see the dust, when you actually look through the viewfinder. Some people have smeared some silicone around the viewfinder or sent it to Leica for service. Again, it's a trivial problem.
I've had my M6ttl now for about 12 years. It's not as pretty as an MP, but has never failed me and I carry it with me every day. TTL flash support is a huge help, when you do have to use a flash and the M6ttl is the only mechanical M to offer this feature.
I simply love the fact that I don't have to rely on batteries, for the camera to take pictures. Batteries always seem to run out at the wrong moment and are susceptible to the cold. AE can be a 'picture saver', but I can live without it.
So, for me the choice is the M6ttl. Get the flare fix for the viewfinder and you will have a companion for life.