Should I use a tax-deferred or Roth 401(k)?

This question can be very complicated and there are a lot of factors that go into it. However, a good general rule of thumb is to use the Roth option in any year when your income is significantly lower than your peak earnings years (think residency, fellowship, working part-time, sabbatical etc) and use a tax-deferred account preferentially during the peak earnings years.