@JaX
Fundamentally, the problem with both models is that nobody knows all of the contributary factors. Of those they do know of, or can guess at, there simply isn't enough information to make a reliable estimate, so it comes down to conjecture.
So far, the only model of life we have is terrestrial and the only place we know it exists is right here on Earth, which depending on which factors you take into consideration is either a quite probable planetary configuration all the way down to an almost unique one.
Therefore, finding life elsewhere in the solar system alone would make a huge difference to our baseline knowledge about the prospects of finding life anywhere else. The spectrum of environments in the solar system is probably far better represented in the local universe we know about than just our wee green belt bit.