Roth vs. Traditional, regardless of if your account is an IRA, 401K, 457B or other generally boils down to what your marginal tax rate is now vs. the tax rate upon withdrawal.
If you think your future tax rate will be higher on withdrawal, then you pick Roth. If you think your future tax rate lower, then traditional makes the most sense.
This being an early retirement forum, many of us are projecting that on withdrawal, we'll be taking out quite a bit less than we're making during the accumulation phase. If I am retired and therefore have the choice, why would I create 100% of my pre-retirement income by withdrawing from accounts when I am spending only 50% or less than that amount? If my income is 50% of what I'm paying today, what will my tax rate be on that income? Not sure, but I'm personally betting lower than my tax rate today.
If your spending is low enough, it is possible to never pay taxes again:
http://www.gocurrycracker.com/never-pay-taxes-again/. If your marginal tax-rate today is 0% or negative, then Roth for new contributions is a slam-dunk. If your marginal tax rate is positive, then there is a decision to make.
Like everything tax related, and particularly with not knowing what the future holds, there is some nuance here and everything is specific to your situation.
Other factors include:
1. Can I even deduct the tIRA contribution? If not, then a Roth IRA, either regular or backdoor if necessary, is the obvious choice there.
2. Have I already filled my future low-tax brackets with prior contributions and the expected earnings on them? If so, maybe it makes sense to pay tax today and go Roth.
3. How do RMDs affect this decision?
4. How do tax-credits and other deductions affect this? Correctly calculating marginal tax rates is often not as easy as you think - traditional 401K contributions can qualify you for tax credits like the
Earned-Income credit and Saver's credit (even traditional IRA contributions help with the Saver's credit), so the marginal rate can actually be shockingly high at low incomes. For example, if your (2016 filing Single) AGI is 18,501 and you have at least $1 of tIRA space left, your Saver's credit can jump from about $400 to around $850 with just 1 additional dollar put into the tIRA - the marginal rate on that dollar is 45,000%!
5. Will I be retiring to a state with significantly different income taxes? A lot of people retire to places like Florida that have no state income tax. If I am earning in a state with an income tax, and planning to retire to Florida, that pushes me even further towards Traditional and not Roth.
Anyway, clear as mud, right? A lot of people draw the line between the 15 and 25% brackets. For an aspiring early-retiree on a middle-to-high-income, Traditional usually makes more sense (
http://www.gocurrycracker.com/roth-sucks/), but you need to consider your individual situation and make the call.
Here's the good news: Roth vs. Traditional is a question of different levels of good for most people. You're 90% of the way by asking this instead of "IRA vs. New Truck" - so get the money into a retirement account and working for you ASAP!
