First of all, there is science-based evidence for both quantum and string theory, so at this point it is up to the scientists to follow present and future evidence to try to find the scientific “truth” of the matter.
Yeah, not so much.
There is a lot of solid evidence in favor of
quantum mechanics. However, there is no evidence for string theory, m-theory, loop quantum gravity, or numerous other attempts to reconcile QM and gravity. Some physicists were so upset over the funding and attention given to string theory that they attacked it as being unfalsifiable, and therefore "not science." E.g.
https://www.scientificamerican.com/article/is-string-theory-science/
Even when talking about QM, there isn't a strong consensus yet on which interpretation is correct. Copenhagen is (afaik) the most popular, with Everett's many-worlds theory, Bohm's deterministic views, and others also holding some sway.
What information would you need to “make a specific decision” and from whence and how would that evidence be obtained?
I'm nowhere near adept enough at high-energy physics to provide a
specific answer to that question. What I do know is that because gravity is exceptionally weak, and because we're likely dealing with the behavior of subatomic particles at or smaller than Planck scale, it may be impossible for human beings to run experiments which explain exactly how QM and gravity function together.
At best, we might be able to see which theory is more consistent with, say, obscure cosmological data. But at this time, there is nothing humans can do which will provide a definitive answer, and that could be a permanent limitation.
We should note this is not the only such dilemma humans face. We could be here all day with examples where the data we'd want in order to be certain simply isn't available. E.g. we currently don't have a machine that can detect the mental intentions of an organism; we can only guess. We will never know why Beethoven went deaf, or the name of the first human, or who invented bread. Some of these unknowns are important, some aren't, but... It really shouldn't be all that revolutionary to point out that uncertainty is simply a part of life.
Supposing is fine, but for an atheist, there needs to be more.
Uhhh... We're talking about agnostics, not atheists.
