320 kbps mp3 vs m4a

If you look how the compression algorithm works AAC has a bit greater dynamics range than mp3. So theoretically at the same bitrate the AAC should sound a little better in dynamics or very silent passages.
However at 320k I doubt you would hear the difference on a good track. I can hear no real big difference 192k and up in AACs and mostly use 256k AAC or 320k mp3 myself. (The 2 should be roughly the same quality even though the AAC is 256k.)

Yes. AAC is the name of the codec (Advanced Audio Codec) and m4a is the file extension (and is actually meaning: MPEG4-AAC, since AAC is recognized by the Motion Pictures Expert Group to be included into MPEG-4 video containers and was actually first developed for this purpose)