My understanding is that the result is identical, but Mixdown takes the new stereo track to its own page in the Editor (with spectral view), whereas bouncing keeps it in Multitrack so you can see it in relation to everything else in Multitrack.
I was wondering, what's the difference between bouncing down several tracls in the multitrack as opposed to mixdowning the those same tracks using the master output?
If you bounce individual tracks, you get all the clips on that track bounced into one file on a new track - this doesn't go via the mixer, although crossfades and automation are rendered. If you bounce the Master track, on the other hand, then everything feeding it (including busses) gets included, so yes it looks like a mixdown, only it stays in Multitrack and is automatically saved in the bounce folder associated with the session. Pays your money, takes your choice...
Also if you Bounce the Master output this will include any effects such as a Limiter across the overall mix.
Ha! I knew I'd be wrong! Let this be a lesson to me.