I have all three (and a little MP-3 which isn't really relevant, but hey). Out of all of them, the first to fall would be the M8. I think that digital is really the future of photography, but for the M-series, the future isn't there yet. Mine's been beset with problems, but my biggest and most unforgiveable gripe is that I cannot use my beloved lenses as they were intended.
So: M7 or MP. The answer to this is "it depends". I love them both dearly, and in many ways they're very similar. The MP feels marginally better balanced due to less height, and its construction feels a good 20% better (if you'll allow me the luxury of a numerical approximation). I never really use the AE on the M7 - the job of adding shutter-speed to aperture and focus is hardly a chore, and the manual metering far more accurate. However, I have grabbed a few shots with the AE that I would never have been able to with the MP and exposure compensation on the M7 can often come in useful...
Other than that, I think the shutter-speed dial is the final issue. If you're used to shooting with an M3, it may be a good idea to stick with the MP as if you're shifting between the two, you may find yourself getting increasingly confused with an M7...
So I guess that's it. Either of them would be a great choice but...
...through a process of eliminiation: get an MP.
