The main difference between a ROTH 401K and a regular 401K, is that the regular 401K is tax deductible NOW. So when you retire, you will be paying income taxes on the both the principal and the earnings in the 401K. In the ROTH 401K, you do NOT get a tax deduction now, but when you retire, you can withdraw from the ROTH 401K without paying any taxes. Not on the money you put in, and not on the appreciation of the account. They both have their plus and minus. If you think the tax rate for yourself will be higher now then in the future, you should use the regular 401K, if you think your tax rate will be higher in the future, then you should invest in a ROTH 401K. If you aren't sure, you should consider putting money into both.