Has other information on the M5. It spot meters TTL, but the shutter operates without batteries, so you only lose the meter if your batteries go. It sold poorly when new, so is somewhat rare, especially in chrome. The references to 2-lug/3-lug are about strap lugs. The 3-lug version can hang vertically, which may not seem like a big deal, but is very nice. It lets the camera hang off to the side in the crook of your elbow, almost hidden, and better protected, and it doesn't swing forward when you bend over. I love this in my Leica CL and Bessa rangefinders. You might be competing with collectors to get an M5, which drives the prices up unreasonably relative to other models. It appears to me that pressure from new Leica mount rangefinders from other manufacturers has recently driven used prices down a bit.
You can also get a nice sturdy aftermarket rewind crank for the MP, which makes it faster than the knob, and less different from the other models in that regard.
I have read that some versions of the M6-TTL eat batteries quickly, which might vary with how you handle them (putting them away switched on). So if you get an M6-TTL, carry extra batteries.