Which Version?

file size: m4a < m4b < m1 < m2.
quality: m1 (VCD-quality) worst, m2 (DVD-quality) best. m4b, however, has practically the same quality as m2.
CPU demand: mpeg1 < mpeg2 < mpeg4, under comparable conditions. older machines may have trouble playing mpeg4.
software support: mpeg1 has the best support; mpeg2 is next; mpeg4 is least well supported.
tips for getting mpeg2 movies to play.
tips for getting mpeg4 movies to play.

So,

if you have a fast machine and can play mpeg4, get one of the mpeg4 versions.
if you have a fast network and a reasonably fast machine, get the mpeg2 version.
if you have a very old/slow machine, the mpeg1 version might be your only choice.