Terra, we use Latin a lot anyway so might as well.
Earth, lets keep things traditional.
Holy Terra, to be precise. Heretic.
Terra1. The second planet we colonize will be called Terra2, and so on...
I don't really have all that great of a reason, it's just that Terra sounds so much cooler than "earth", and it fits in with the rest of the planet names. Heck, let's name it Gaia. That's even cooler.
Terra would be neat.