Option Number One is Europa, the favored satellite son of many exobiologists and even Arthur C. Clarke himself.  While distinctly non-Terran, huge sub-surface lakes probably heated by tidal stresses, and even an extremely tenuous oxygen atmosphere make it a leading contender.  Hot water and even some air?  Is there a more likely life-site without tiny bacteria-sized jacuzzis?

Number Two is Titan, a very-Terran option whose surface lakes, shorelines, seasons and relatively thick nitrogen atmosphere mean it's viewed as an early-model Earth.  And 100% of all know Earths have awesome life on them!  The significantly lower temperature is a bit of a stumbling block (it's ten times as far from the sun as us), but the possibility of subterranean microbial life - or even a prebiotic "Life could happen!" environment - would be a massive result.

Remember, most of space is empty.  Either "not enough there to even count as dead" or "hard radiation sterilized space that would make a bucket of bleach in a blender look like a life-form holiday home". There's life out there somewhere, and anybody who says otherwise simply doesn't understand how big the universe is, but having three such suitable environments in one (stellar scale) space?  The solar system is three winning lottery tickets delivered by a trained unicorn and we'd be fools not to collect.

Right now having to choose is hard.  But being able to choose is incredible.

Posted by Luke McKinney.

NASA, ESA to decide on 'life' mission http://www.abc.net.au/science/articles/2009/02/10/2487461.htm