The differences between a Leica M6 (classic) and an MP:
1. The MP has a brass top; the M6 is zinc alloy;
2. The MP has the “Leica” script engraving; the M6 has no engraving (early M6 camera tops from Wetzlar are engraved “ERNST LEITZ WETZLAR GMBH”);
3. The MP has the classic M3/2 RW mechanism with vintage style knobs; the M6 has the M4 style with the slanted crank with M4/5/7 style knobs;
4. The MP has the RF condenser lens (taken out in the M4-2 run); the M6 does not.
5. Different body coverings (neither is extraordinary);
6. The black MP cameras are painted (and the paint is ‘designed’ to rub off); the black M6 is anodized black chrome;
7. The MP has more modern meter circuitry and the meter may be a stop more sensitive in low lighting conditions (replacement M6 circuit boards can no longer be had);
8. The MP generally has a smoother film advance mechanism compared to an M6. This is a result of some fine tuning;
9. The MP is fitted with a separate eyepiece with a plastic cover installed. Every M camera prior has the eyepiece molded into the body with a diopter that screwed in. The new eyepiece was not originally sealed and allowed dust into the finder over time; Leica now seals the eyepiece at the factory, but it is still an inferior design compared to any that preceded it.
10. The hot shoes are different – neither is better or worse than the other;
11. The MP has coated RF windows – the M6 does not.
The shutter sound differences noted by some are partly due to the differences in top plate metals. It should be also noted than shutter sounds may vary from camera to camera (two M6s, for example, might sound different due to differences in wear, lubricants, and micro variations within tolerances). I have seen this often.
I have owned and used both. In spite of the differences, functionally they are the same camera with the same build tolerances; both have the same shutter mechanism and the same RF (with the condenser lens added in the MP – a ~$200 upgrade through DAG – more if done through Leica).