This decision mainly boils down to how you want to deposit money into the account and how you want to withdraw money. Let's start by investing money today. If you prefer to pay taxes now and get them out of the way, or think your tax rate will be higher when you retire than it is now, choose a Roth 401 (k). A Roth 401 (k) is a relatively new addition and allows for a different type of tax relief.
With a Roth 401 (k) plan, you'll make after-tax cash contributions, so you won't get a tax break today. In exchange, any money you withdraw during retirement will be tax-free. If you expect to be in a lower tax bracket when you retire, a traditional 401 (k) may make more sense than a Roth account. However, if you're now in a lower tax bracket and think you'll be in a higher tax bracket when you retire, a Roth 401 (k) might be a better option.
Choosing a Roth 401 (k) or a traditional 401 (k) may not be an either-or decision. If your employer offers both options, you can contribute to a Roth 401 (k) and a traditional 401 (k). Your employer can also match both options, but the funds from your traditional 401 (k) plan go directly to your account, while with a Roth 401 (k) plan, they are deposited in a separate tax-deferred account. Meanwhile, converting a traditional 401 (k) into a traditional IRA doesn't help you avoid RMDs, and you can't convert that account into a Roth IRA without incurring high taxes.