If you actually invest the tax savings, it is almost always better to go with the traditional. There are a few occasions where a Roth is ideal:
1) Taxes increase by the time you retire. How much they would have to increase is dependent on many variables. Also, the likelihood of them increasing probably depends on your income. Taxes are more likely to increase for high earners than for lower and middle class. Unless you plan to withdraw a lot of money during your retirement years, I wouldn't worry too much about this.
2) Expenses in retirement will exceed income during earning years.
3) You want to make things easier for heirs. The taxes involved with a traditional 401k/IRA inheritance can be tricky. That isn't the case with a Roth.
4) You have no income tax to defer. This happens for me. After loading up 401ks and receiving various credits, we don't pay income tax. As such, we fund a Roth IRA.
Also, it's always a good idea to have a mixture of both, as a hedge. If you have too much in traditional 401k/IRA investments, required minimum distributions might eat your nest egg alive when you turn 70.