I agree. It is not something we can decide or not. If I can't prove something exists and also I can't prove it doesn't, I simply don't know if it exists or not. I choosed undecided but I mean "I don't know.".
I don't get the question. Why is it necessary to "believe" in aliens? We're not talking about a Supreme Being, whose existence cannot be proved or disproved. Either aliens exist or they don't, and my "belief" in the matter is irrelevant.
It's like asking, "Do you believe in somebody named Terry Finch who lives in Calgary?" Either Terry Finch exists or he doesn't.