I'll give you of what I think of them.
So as of right now...
MG has 8 player support, better multiplayer support/gameplay, better map and g3d editors, particles (so more fancy), font "changelings', font color, and more.

GAE? Tons of useful features that can make your day very happy.

like save game, auto repair, patrol and guard
supposedly water units, etc. etc.
Glest is the "base" of megaglest and GAE.
So anyways, basically just practice and practice. That applies to everything.
