The multiple pass method produces a higher quality file, but in most cases I recommend that people just use the single pass export, as it's a lot faster. With regards to FPS settings - it depends on a number of things, for example, what the FPS of your files are, or perhaps what you're exporting the mix for (NTSC (29.97), PAL (25), etc). I'm not sure that it's possible for me to estimate the export time - though it's something that I'll look into.
