It would be the discovery to put all other discoveries in the dark!
It depends. It'd be a major anticlimax if it turned out to be identical to something already known on earth.Even if they did find something we don't yet know of, I don't see how its any different to finding a new species on our own planet. Its already known that there are sulphur-based life forms here. They were found a few years ago in South Africa (I think).