I have absolutely no idea where I stand on this question anymore. It's so tough. I'm almost more interested in whether or not abiogenesis occurred twice in our solar system. I think that would have more relevance to our understanding of life in the universe. Apparently the ESA 2016 and NASA 2020 missions will be better equipped to check of this. I've also heard a few people saying that our best way to find out would be to bring back large samples of Martian rocks. Let's go Mars-One! Exactly! I would hazard a guess that we are very lucky to have two planets in the same solar system with the potential to support complex life. That's probably very rare.I still give better than even chances that there is microbiotic life on Mars.
I was a bit disappointed that Curiosity wasn't more equipped to search for signatures of life.
Ultimately, it might come down to human exploration to really determine if life is there.
however, it's almost dumb luck that we can even entertain the notion.