Aliens might very well exist but almost certainly not in the way we typically think of them (i.e. humanoids with flashy advanced technology who abduct people). I'd be surprised if there wasn't some kind of life on other planets somewhere in the universe, but I'd be much, much more surprised if there was and it resembled humans at all.
My point was that there's no direct evidence for the existence of aliens. We can theorize all we want about whether they exist or not, but there's really no way to tell. The fact is that aliens do not affect our lives (unless they are inside the solar system they would have to life many thousands of light years away, so we couldn't communicate with them), and I think God's really the same way. For everything where you could say "God must have caused X", there are other explanations, and even if you think God exists and watches over us you have to admit he basically never intervenes in human affairs (see the fact that plenty of Christians get terminal illnesses and cancer and are NOT saved, while on the other hand powerful, evil, non-religious people are allowed to live to old age and perpetuate their atrocities for decades). The best we have on both counts is that some people say that certain things seem like they must be caused by God/aliens, but that's not proof in either case.
What this leads to is the fact that I have a problem with anyone who says belief in God is "obvious" or "logical" or anything like that. [Important side point: I don't actually care what you believe in, you're free to believe anything you want, just don't ask or expect me to agree with your belief if there is no evidence to support it.] If you look at it from a scientific standpoint the evidence (if you think there even is any) is not conclusive... and if you really think it is, I think you ought to believe in aliens and bigfoot and all of that as well, because the evidence is just as (un-)convincing there too. There is just ONE difference, and it's critical: many people are brought up from childhood believing in some kind of deity, so they don't really get the chance to decide whether to believe or not. Of course it's "obvious" for them, but you can raise children to believe pretty much anything (see: Santa Claus).