If your choices are Roth or not 401k, you want to do not if your tax rate is currently high, and roth if it's currently low. Both rates relative to your estimated rate when you plan to take distributions.<p>Note that the more of each flavor you have, the more it pushes you to the other flavor. If you have a large regular 401k balance, your taxable income will be high in retirement, so you may not want to defer more taxes until retirement age. And if your Roth balance is large, you have access to a lot of untaxable income, so you may want to defer more taxes until retirement.<p>Otoh, if you expect to have taxable invesments, and retire early, living off of taxable, you may have the opportunity to convert to Roth at low rates.<p>It kind of depends on how much you want to optimize.<p>Also, some companies offer an after tax 401k, some with automatic conversion to Roth. Then you can take the limit of tax deferred contributions <i>and</i> contribute a significant amount to (effectively) Roth and have both.<p>Oh and one more thing. The contribution limit is the same number of dollars, but Roth dollars are more valuable than traditional dollars, so Roth contributions gets you more money into tax priviledged accounts (of course, Roth contributions are more expensive, too).