In terms of spatial resolution, none. In terms of temporal resolution, you'll lose 6 frames, but these frames are interpolated from the 60i footage to start with, so you really don't lose anything, as nothing is "tossed" as opposed to "blended."
I've only done limited conversion (for testing only) using Magic Bullet in Vegas 6, and personally, I prefer Vegas 6, although some of the Magic Bullet looks are stunning, if you can stand the render times.