When to choose Terrans and how to make the most from them
Terrans will become more attractive to pick, the more of the following are true:


You are able to pick starting positions, where you can use your terraformer. Ie. the purple planet is next to or close to a blue planet. Preferably both your starting planets.
During the first or second turn, you will get bonus points for building your Planetary Institute
During the game, you will get bonus points for colonizing Gaia planets.
The round booster giving points for colonized Gaia planets is available.


During the game, you will probably want to:


Get your Planetary Institute on the board early, enabling your special ability to obtain stuff for free during the Gaia phase.
Get hold of the tech tile, which allows you to take a special action giving you 4 power.
Use this tech tile in combination with your terraformer. Use the terraformer to move a bunch of power tokens from your 1-bowl into the Gaia bowl. In the next turn, these will return to your 2-bowl. And with the special action, your will quickly have 4 power pushed into your 3-bowl. Once they are spent, one of your last actions that turn, should be to use your terraformer, thereby emptying your 1-bowl. Clean-rinse-repeat.
If one of the late game turn bonuses is the 4-points for colonizing Gaia Planets, you may want to go deep in the Gaia tech track, enabling you to terraform three purple planets on the turn just before that, and then follow up with colonizing three Gaia planets and netting 12 points.
If it suits your plans, you may also want to pick up the tech tile, that gives you 3 points every time you colonize a Gaia planet. The earlier the better.