I can see Arum's idea of the Earth mark being better. I just run through about 10 games against AI3 in the trainer, 5 with Earth Mark and 5 with Entropy. Although 10 games is not really enough to make a sure prediction, it takes somewhere around 2 more turns with Earth than Entropy.
Now I would say if you are in a situation where there is an Oty, Scarab or some mutant which enjoys sparks for breakfast, lunch, and dinner, I would lean toward having an Earth mark.