Putting money into a Roth IRA or 401(k) will save you money if your taxes this year will be lower than your taxes in retirement.
If you're young and just starting your career, there's a good chance of this in general.
Even if you're not, most people are anticipating tax increases in our nation's future If you have a lot of expenses one year, e.g. for medical reasons, you may have to realize a significant amount of income that year, so having it be Roth money instead of regular can help avoid higher taxes.
See also the Wikipedia retirement-savings matrix.
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Another consideration is that you are going to wind up with money in the "regular" 401(k)
no matter which one you contribute to.
The employer match can't go into the Roth 401(k).
So all employer matching funds go in with pre-tax dollars and will be deposited in a normal 401(k) account.
Edit from JoeTaxpayer - 2013 brought with it the Roth 401(k) conversion the ability to convert from the traditional pretax side of your 401(k) account to the Roth side.
money.stackexchange.com/users/149/JohnFx
If you are in the 15% marginal tax bracket, I'd agree, no harm in using Roth.
If you are in the 25% bracket or above, it would take quite a bit of savings to retire in a higher bracket.
I can't know your entire situation, but the average person is barely saving enough to be in the 15% bracket in retirement, let alone higher.
money.stackexchange.com/users/187/JTP - Apologise to Monica
Also, consider the possibility of early withdrawal penalties.
Regular 401k early withdrawal (for non-qualified reasons) gets you a 10% penalty, in addition to tax, on the entire amount, even if you're just withdrawing your own contributions.
Withdrawing from a Roth 401k can potentially mean less penalties (if it's been in place 5 years, and subject to a bunch of fine print of course).
money.stackexchange.com/users/1227/Joe Enos
As fennec mentioned, it is about your tax rate today vs your tax rate when you retire.
One of the big issues is whether you have a lot of tax deductions today that will disappear when you retire, which is likely.
These include dependents (aka kids) and your home mortgage.
For a person with kids and a mortgage, it seems likely that the Roth would be beneficial.
If you do not have a lot of deductions, you might be ok with the regular 401k to help reduce your tax burden today.
money.stackexchange.com/users/338/Swoop
I would look at it as more of a balance in general assuming you have the ability to contribute pre or post tax money and you expect your retirement tax rate to be about the same.
Pre-tax money gives you flexibility of withdrawal during retirement.
Say one year you have unusually high expenses you can pull money from your taxed accounts until you hit the top of your target tax bracket and then pull the rest out of already taxed accounts.
Note that if you have the same tax rate today as when you retire.
The traditional account will net you slightly more money, but not a huge amount.
money.stackexchange.com/users/1873/stoj
My employer allows new contributions to my 401(k) to go in as Roth (post-tax) or "regular" (pre-tax).  What should I consider when deciding how to distribute my contributions between the two?
What factors should I take into account when deciding how much of my 401(k) contribution should go into my Roth 401(k)?
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<united-states><401k><contribution><roth-401k>
money.stackexchange.com/questions/2891
money.stackexchange.com/users/1226/Daryl Spitzer
Take the Roth option if your taxes this year will be lower than your taxes in retirement. It could also mean less penalties if you make an early withdrawal. However, if you don't have a lot of deductions, then the regular 401k might be better to reduce your tax burden.  There are number of aspects to consider when deciding how to distribute your 401k contributions, including your current and expected tax rates and what potential withdrawal penalties there are. If you do not currently have a lot of tax deductions, you may want to use your regular 401k to reduce your current tax burden. Additionally, a regular 401k provides greater flexibility on withdrawals in retirement.
When considering how to distribute your contributions, you may want to consider your tax rate and how it may change in the future. You may want to consider what early withdrawal penalties there are in each case. If you do not have a lot of tax deducations, you may be OK with a regular 401k to reduce your tax burden. A regular 401k gives you more flexibility on withdrawals in retirement.