In the popular imagination, the Big Bang is a great explosion at one time there was nothing, then matter erupted into previously empty space. However, the Big Bang is the beginning of spacetime itself, not an event in time.

To talk intelligibly about modern physics, we have to admit the possibility of uncaused events.

Physicists use 'God' as a metaphor more often than other scientists-- especially in popular writing, but in the technical literature as well. Of course, this is just a metaphor for order at the heart of confusion. A rational or aesthetic pattern underlying reality is far from a theistic God.

Which is better: to achieve Nirvana, or become a Boddhissatva?

Attaching a Creator to the boundary is metaphysical skullduggery.

Creation out of absolute nothing is a metaphysical quagmire for theists anyway, since nothing must at least have the potentiality for becoming something. Since theists are stuck with potentiality, it might as well be something like a quantum vacuum.