Good debate.
MG (and other glest) are free and open source, which is a huge deal IMO. You also mention the gameplay, which is it's real strength I feel.
But you don't seem to mention the mods. For me this is the huge success in glest. If you haven't already, I suggest you try some mods, there are many depending on your taste. Apologies if you do cover that but I missed it.