Do we want to be Bayesian about it? Of course we do. Let's imagine two universes. One formed spontaneously, one was created. Which is more likely to occur?

It isn't obvious that this is at all meaningful, and gets quickly into deep issues of anthropics and observer effects. But aside from that, there's some intuition here that you seem to be using that may not be shared. Moreover, it also has the weird issue that most forms of theism have a deity that is omnipotent and so should exist over all universes.

Note also that the difference isn't just spontaneity v. created. What does it mean for a universe to be created? And what does it mean to call that creating aspect a deity? One of the major problems with first cause arguments and similar notions is that even when one buys into them it is extremely difficult to jump from their to theism. Relevant SMBC.