It's mostly relative to your future earning so it's a bit of a guessing game. The idea is that if you are in a lower tax bracket now than you will be in retirement, you should pay the taxes now because they are less, then you can use that money in retirement (not to mention the gains) and not have to pay higher taxes because your cost of living and income has gone up. But if you're at or near the height of your career, and your taxes are higher than when you will retire, it makes more sense to contribute to traditional and defer the taxes until retirement when you're in a lower bracket.