Actually, there's just one thing about your concept of an eternal universe which needs addressing: the "expand/deflate thing" probably isn't true. The universe isn't slowing its expansion, it's speeding up. That means it will never shrink back to a Big Crunch, and this particular universe isn't cyclical in the way you imagine.
That doesn't stop the universe or multiverse from being eternal in other ways, so the question is still valid.
The law stating that matter and energy cannot be created or destroyed does suggest all by itself that the universe is eternal. This does not necessarily mean, however, that all possible universes have already happened. It may go along some form of cycle. To demonstrate, take a calculator, enter 1 and then multiply it by -1. Then multiply the answer by -1, over and over again. You'll only ever get -1 and 1. You certainly won't get all possible numbers.
You're right, we don't actually know whether the universe is eternal. Even if it isn't, though, it doesn't guarantee the existence of a god or even a First Cause. This is because of two things:
1. This universe's time is highly unlikely to have been entirely linear and consistent around the Big Bang.
2. The probabilistic behaviours of particles according to quantum mechanics have no discernible cause at all. It's just possible that not everything in the natural world even requires a cause.
With time and causation themselves in doubt, the First Cause argument is a bit of a moot point.
As for which is more plausible out of an eternal universe and an eternal god, that's up to each person. I think of two things: the fact that before the Big Bang theory the most popular one was an eternal "steady state" universe, and the fact that an eternal, uncaused, uneducated god makes eternity all the more mysterious and unfathomable. Whatever the universe got up to in all that time, at least we can imagine some of it. With a god, we haven't a clue.
- SmartLX