I like this question, and I'd like to answer it as well if I might?
What is your main reason for ---God might exist?
First off, surely I (and life forms) do not (and most likely cannot) know everything there is to know. For that reason, I cannot rule anything out with 100% certainty regarding the existence of some god. Secondly, I think this question depends on the definition of god. What god is proposed to be influences my likelihood of believing it exists. Is god just the pool from which life is dispensed? I am more likely to accept that that god being defined as a human-like character with emotions, because to me it seems unlikely.
What is your main reason for ---God might not exist?
Many of the same conditions apply here. Can life itself know what god is? I am not sure, I am prone to say no, and for that reason I choose Ockham's razor when waging my 'bets' on the origin and meaning of life. I suspect god stories are lies, misunderstandings, and twists of reality (some benevolent of course; a mark of individual perceptions and differences). If a god exists, I feel it likely does not relate to life, in the way Deists believe.
Again, the definition of god is of utmost importance here. I am an atheist of gods as we know them. I've seen this credited to numerous people so I will not give incorrect credit: “We are all atheists to other religions, we [atheists] just take it one step further.” This strikes me. No major/common religion has defined a reasonable nor plausible god, in my opinion. I suspect humanity knows this, and for this reason, recycles gods, renders some null, and carries on. Mind you this process is slow, and humans are prone to not seeing the big picture. 2000 years is forever to us, but it is surely not forever to time.
Evolution is also a main reason I do not find sufficient evidence to support a god as we know or have known it. In my opinion, the process of evolution screams out loudly and clearly that this world was not made just for humans, and it was not made nor designed with an intelligent plan. Evolution seems like a stronger atheist than even I am, because I still make way for the unknown, and our limited capacities.
Thanks for the thought provoking question. This response is just a work in progress, open to changes and edits, knowingly futile but ultimately challenging and, in my opinion, a neat part of life.