Eh, naming the planet after a greek god feels weird to me (especially since we named all the other planets after the Roman gods), and it would be even weirder to give it a girl's name. I've never heard of anyone (sane) named Earth, so to me it's a nice, neutral name. Luna is the name of the moon, but since it's our one and only moon we just call it "the moon." Same for Sol, our sun.
Besides, Earth just sounds right. It's a good, strong name which fits well for the densest planet in our system and also the largest terrestrial one. Most of our planet is made of fucking IRON. That's fucking hardcore. Terra just sounds wimpy, like every other Latin word that doesn't end in "-us."
Wikipedia:
It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation, and is probably the only one with active plate tectonics.
That's right. EARTH, ************!
(Notice how TERRA, ************ just doesn't have the same punch.)