Apples and oranges, depending on the type of pictures you take.
I use my M6 often and my F is collecting dust (o.k., my SLR needs are taken care of by other cameras).
Old story: for what an M does well, it can be tops. For what it can't do, an SLR is needed. Of course, ideally you should have both! ;-)
For a trip I'd very probably take the M for its size & weight advantages, unless I really needed longer teles (not too likely on a longer trip).
M6 + 21 +35 & 90mm : 1080 grammes
F(w Ftn) + 20 +35 & 105mm : 1879 grammes
The Ms lenses are also better....