One other thing to consider...
Effective tax rate and marginal tax rate...let's take Tuna's case where his effective tax rate is 4%. Due to the bracketted nature of the system, the first 18k or so he makes is taxed at 10%. 18k to 76k at 15%. 76k to 153k at 25%. And 153k to 233k at 28%.
https://www.bankrate.com/finance/taxes/tax-brackets.aspx
This is before deductions, of course. If he made, for example, 75k per year (to keep it simple) then he pays 1800 in taxes (10% of 18k) plus 8600 in taxes (15% of 57k) for a total of 10,400 in taxes, or an effective tax rate of 14%. Using his _real_ effective rate of 4%, he's paying only 3000 in taxes due to deductions- kids, mortgage, gambling losses, whatever.
HOWEVER- if he were to make $1 more in income, let's say- that $1 will be taxed not at 4% nor even at 14%, but at the full 15%. And if his income is over 76k, then that next dollar is taxed not at 4% nor 14% nor even 15%- but the full 25% of the next bracket!
I guess I would look and see what the adjusted income is, and play with the numbers to see what he actual dollar change in tax burden is given the two options (Roth 401k vs traditional 401k). Then play some prognostication and think about what the income will be in retirement, using the same brackets.
Of course, the rules might change. Eventually some leader might come along who decides to balance the federal budget. Taxes are not likely to go down any more in our lifetime. They have a very real chance of going up, I'd say. On the other hand, the odds of the government one day decreeing that all Roth future earnings are suddenly subject to taxation are very low, I'd wager.
My best educated guess is still slightly in favor of the Roth 401k over the traditional 401k. But it's not something I'd lose any sleep over.