As per the argument in Ward & Brownlee's Rare Earth, it's easy to get a roughly earthlike planet filled up with primitive bacteria. It's very, very hard to get and sustain any more complex ecology. In the entire history of human space exploration so far, the worlds with ecosystems in any sense comparable to Earth's can be counted on one hand. In practical terms, therefore, the available earthlike worlds can be divided into categories: sterile (from heat, cold, etc.) and vibrantly toxic, with poisonous atmospheres and a biosphere full of sludge.
There are two big camps in the imperial institutions that deal with colony development: the Wets and the Drys.
The Wets favor terraforming of worlds with biospheres. They differ amongst themselves about strategies, but they share the conviction that it's better to start where life already is and work on making it more suitable for humanity. Genetic subversion, aggressive terro-colonization, multiphase niche transformation and folding, these are the things technical Wets argue about. The field forces start with sealed bases and work from there to carve out living space. Risky techniques can breed safe environments in 50 years or less, at the risk of unleashing accidentally lethal species, while safer layering methods make a world tolerably habitable in 2-300 years.
The Drys like habitable worlds as much as the next person, but prefer to start where life isn't. The Soft Drys are terraformers, introducing Earth-style ecosystems piece by piece along with atmosphere and the other appointments of a comfortable life. The Hard Drys don't aim for ecosystems. They work with asteroids, gas giant moons, and even interstellar debris with convenient D-drive access: anything with extractable resources is some Hard Dry's baby. Where the dream of the Wets is a sky full of Earths, Drys tend to think in terms of a sky full of Dyson spheres (the original scheme of many, many, many bodies orbiting a star so as to capture all its energy, that is, not a hard shell).