I used to have an M3, and now have a IIIf. I find the M3 unquestionably easier to use, the finder is a great deal better, it's easier to load, and you don't have to faff about with the two shutter speed dials, focus the RF etc.
However, the IIIf, with collapsible Elmar will literally fit in my jeans pocket, and it's not even that big a pocket. The M3 with Summarit (obviously you could use a different lens) is really quite a lot bigger. For all the talk about Leicas being small, they are not that small really until you go back to the LTM models. My M3 with lens was bigger than my Zeiss 6x6 range finder, which is crazy really. The IIIf however is even more portable than a Rollei 35, as the latter's "boxiness" makes it difficult to pocket.
If you don't mind the extra size, M3 is the better camera in my opinion. If you want as small as possible, and of course the old-world charm, then a IIIf is just lovely.
Finally, the IIIF with lens cost less than half what I sold my M3 with lens for.