My 401(k) is a traditional one. My employer contributes 6% of my salary to it. I can contribute whatever I want up to certain maximums ($22,000 which includes 'catch-up' provisions for those of us over age 50). I have the choice to add funds to the traditional 401(k) or to contribute to a Roth IRA or both.
I have always chosen to stick with the 401(k), largely because it reduces my taxable income.
Still, it is attractive to think that with a Roth, I could withdraw funds or not as I choose (whereas I have to start 401(k) withdrawals at age 70.5) and that those withdrawals would be tax free. (Not exactly, of course. Taxes were already paid when/if I made the deposits into a Roth.)
But this article convinces me that, at my age, and what looks to be my financial situation when I retire, it is too late for a Roth to help me. Certainly, it would be way too expensive to change over my traditional retirement savings to Roth accounts.
But for those with a 15 year or more timeline to retirement, the Roth may well be the way to go.