You're both right. Yes, we should check very carefully, but as you point out we have more than one body to play with. We should have a clearly defined cutoff point at which we stop looking for something that may not be there in the first place and do other things instead. Otherwise we're never going to get off the planet.

As these things go, Mars is far less interesting to me than Europa. That moon of Jupiter seems to have massive amounts of liquid water, geysers, ice quakes and plate tectonics, (presumably due to) tidal forces from Jupiter, and a thin oxygen atmosphere. Along with Saturn's moon Enceladus, it's the most promising place to look for other forms of life.

http://en.wikipedia.org/wiki/Europa_(moon) http://en.wikipedia.org/wiki/Enceladus_(moon)