How can we determine the optimal destination for a manned mission beyond our solar system?

Question

The 100YSS mission currently includes a discussion about Alpha Centauri (our closest star) as a potential target destination, but is it really the best candidate for the 100YSS?

To narrow the question's scope, I'll add that my own criteria in asking the question make the discovery of extraterrestrial life the highest priority, so I would add:

1. stars with larger numbers of Earth-like planets are better than stars with fewer (in hopes of maximizing the probability of finding life)
2. closer stars are better than farther stars (to minimize travel time)
3. stars with more stable surroundings (like our star) are better than stars with less stable surroundings (like stars in a binary system such as HDE 226868/Cygnus X-1)

Answer 1

Through unmanned missions.

A set of probes of moderate complexity would be able to gather enough data to help making the decision.

I can imagine prerequisites of such probes.

• as it gains speed, it discards trailing segments. Possibly another probe just with such segments is sent. At preprogrammed time the segments activate and start acting as radio relays - we don't need the probe back, just its data, so the "way back home" of the data would be at speed of light, from relay to relay.

• The probe contains a central module with a good spectrometric telescope to provide automated survey of the planets, and a couple landers similar to simple Mars landers, autonomous vehicles capable of airbraking and landing on potentially inhabitable planets. We don't need inhabited, just inhabitable by humans or at least resources-rich so that the actual 100YSS could establish a base of operations.

• The core module picks promising candidate planets using the telescope data, and deploys the landers to these. The probes relay their findings to the core module, the core module sends it back home through the relay system.

• One such probe is sent to each candidate star system.

• As the probes are underway, we use the time to develop actual spaceship, develop needed technologies for easily deployable sustainable base, and so on. We should have it ready once the replies start coming in, then we pick the best candidate planet using the gathered data - if there is any life in given system, a most human-habitable planet in that system. If there is no life in any of them, most human-habitable of them all.