It really depends on your sound system. AC3/2 is Dolby Digital 5.1 and is always my choice. Obviously having only a single audio stream means higher quality for the video on a single layer DVD.
Mpeg audio is mp2 that can be the output from video conversion suites like DVD2SVCD. Any TV series I d/l will get converted to this when I burn them for viewing. Showing the details on my Pioneer DVD Player will show either AC3/2 or Mpeg 2ch.