Both accounts produce their biggest advantages if you don't withdraw funds before you reach age 59 1/2. Withdrawing from your 401(k), whether traditional or Roth, before that age may require you to pay a penalty and additional income tax. There are certain exceptions, such as first-time home purchases and medical or educational expenses. You can also withdraw funds starting the year you separate from service if you turn 55 or older that year, but that lets you avoid only penalties and not the taxes on early withdrawals.
With a traditional 401(k), early withdrawals that don't fall under an exception carry a 10% penalty, and you'll usually owe income taxes on the full amount of the withdrawal. With a Roth 401(k), your withdrawal will be prorated based on the proportion of contributions to earnings in the account. You'll pay only tax and the 10% penalty on the portion of the withdrawal attributable to earnings.
If you make it all the way to retirement age without touching your funds, you can save a lot of money. Withdrawals from a Roth 401(k) come out entirely tax-free for both your contributions and your earnings. You'll still owe income taxes on the full withdrawal from a traditional 401(k), but you won't be subject to that nasty 10% penalty.