Interesting question. I have both F and M3.

For any occasion where the shooting requirement matches the capability set of the M3, I would leave the SLR at home. The M3 is a joy to use, and it travels well. People don't seem to notice it so much, nor do they take it so seriously.

If I needed to do things that a rangefinder cannot do as well, then I would take the F instead. It is far more flexible, if somewhat more cumbersome.

It might be a good idea to visualize the images which must be gotten and decide which gear is the best match for getting them in the can.