You know, I'm really glad I never even had (or rather 'gave' myself) the option of buying an M8 because it is quite apparent from this thread that there are many great things with both to be passionate about. Surely this seems like a tough decision given the great divide I see here. For me, given the option of either, I feel like it's entirely a shooting style issue.
To the average photographer - and perhaps I'm the only one of them here in which case I apologize 🙂 - I say forget about 6MP vs 10MP. Forget about embedded focal lengths in the EXIF. Forget about analog dials vs 7-segment displays. I'd even go so far as to say forget about 'build quality' and 'brand longevity' (most will agree both are built solidly, a great MANY have had perfect R-D1s (not all R-D1 users post on this board), and as long as there is a market - and it looks like it's growing - you will find someone to repair your RF camera).
IMHO, I think the best question to ask is: which camera empowers you to make pictures the way YOU want to make pictures? Will a more film-like camera let you get closer to your subjects? Will being able to keep both eyes open enable you to compose better? Will carrying a camera with a $5000 price tag prevent you from going to certain locations or make you leave the camera at home?
To me it's all about the usability of a camera specifically towards composition. Whatever camera you use, you'll get over the little things like an extra button press or two. Having to wind between photos or not. Having to take off the bottom plate to put in an SD card. Having to double/halve exposure due to your camera's individual quirkiness. These, to me, are trivial matters which, with time, will become second nature to anyone and, at best, a bit annoying. The important thing is that, in the time where you're not annoyed by those little things, you're out making pictures exactly where you want to make pictures, and exactly how you want to make them.
Again, this is what I think people like myself should be focusing on given you could buy both. You pros out there, I'm sure, have a quite different angle, but then you stopped reading this post far before here anyway so this disclaimer is lost. 🙂 And, btw, I'm so guilty of not doing what I suggest above it's not funny. Call it the engineer in me. But oh the bliss to finally let it all go and just get out and work on your PICTURES! : )
Speaking of which...