Believing in extra terrestrials is not as outlandish as it might seem. The universe is larger than any of us can really comprehend, and given the amount of star systems with planets like ours, statistically there HAS to be life out there. However, the likelihood that they've ever been to Earth is extremely small. So while I believe in aliens, I don't believe we've ever seen one.
Where god is concerned, it really depends on your definition of god. If you believe there are beings in the universe that are god-like, then I could understand that argument. But believing that an all-encompassing god exists out there, centralized on Earth for some reason, I feel has much less weight to it. The Earth is proof that life can exist in the universe, when certain parameters are met. So we can reasonably assume, if the same conditions are met elsewhere, life will exist there too. However, without an existing example of a real deity, we can't assume the existence of any other gods.