I think that to some degree, it involves an honest assessment of what you think the future is going to be like.
Not only what are tax rates going to be in the future, but how much income do I expect to have in retirement? Do I expect to be moving to a state in retirement with a higher or lower state income tax rate? Do I think I would be subject to RMDs that kick me into a higher tax bracket? These questions, and more, may influence which way you go.
That said, none of us has a crystal ball or total clarity about our future. For that reason, just as we diversify our investments among different stocks, different sectors, different asset classes... we can diversify with taxable accounts, conventional retirement accounts AND Roth retirement accounts. This gives more flexibility in terms of "engineering" your future income -- to maximize the benefit of lower tax brackets, to keep taxable income just below a certain thresholds (such as MAGI for ACA purposes), to harvest long-term capital gains while in lower brackets and to harvest tax losses to offset other capital gains.