Although I don't own any of the equipment that you're interested in, I have purchased backups for other equipment over the years. The best rule I've come up with is to have the back-up be as near to the original as possible. If you're happy and comfortable with the M3, then get another M3. That way there's no learning curve. I wanted a backup for my Nikon FE-2, and picked up a Nikon FE. Although they're very similar, the dissimilarities would cause me problems at the worst possible times...those moments when there's no time to think, just to react.
If, on the other hand, you're trying to expand your selection of cameras, that's the time I'd recommend looking at other alternatives. (By reputation only, it seems you can't go wrong with any of the cameras you've mentioned.)