Interesting points raised here and there. Again, I guess, it has been proven that 35/50 is a natural and popular dual lens choice. Of the two, I would be inclined to lean towards a 35mm as I prefer the flexibility, intimacy with the subject(s) and - as others have mentioned before - often lack the space or time to step back.
If I had to pick any one focal length, though, it would be a 40mm. It offers a nice compromise of natural presentation and versatility. I also found it refreshing that its rendering deviates from the excessively used 35mm and 50mm perspective.
To that end, I am both glad and surprised to see, that there is so much praise for this 'odd' 40mm focal length that has been 'neglected' by Leica for quite some time now. Both concerning newer lens designs and cameras with 40mm frame lines.