Age. Current marginal rate. Total saved so far. Current rate of savings. Joint or single filer.
These are among the variables that go into making this decision. Without this, my answer is a general response.
In general, you have one marginal rate today. (Unless you happened to be straddling a bracket limit).
In retirement, you have your marginal rate, of course, but also every bracket up to that level. It can make sense to save today pretax to avoid 25%, knowing this money will be withdrawn at an average 10 % or so in retirement.
Edit to clarify to the one who offers comment below to the contrary.
The 2015 taxtable for single filer:
A single person has a combined $10,300 standard deduction and exemption. This means that if he has no other income in retirement, a withdrawal of $47,750 results in a tax bill of $5156. This is an average 10.8% on that withdrawal. It also means that one can save nearly $1.2M before hitting the 25% bracket in retirement. With the numbers I offered, the next $1 is taxed at 25%. In general, if a new worker starts by using Roth, and goes to traditional to avoid slipping into the 25% bracket, they will have a nice mix of pre and post tax money.
In the end, it's not a long term binary choice. Each year, you can decide which flavor or mix of flavors to use. You can convert from traditional to Roth each year to "top off" the 15% bracket, so you retirement withdrawals never push you into the 25% bracket.
Note - the math above tragically ignores The Phantom Tax Rate Zone caused by the taxation of Social Security benefits. For a young person, I don't know that I'd advise counting on this benefit, but if you believe in fairy dust, unicorns, and the like, you should be aware of how the government currently plans to tax you. This situation leans strongly toward the Roth. Until congress decides to use Roth withdrawals as a trigger to tax or reduce your benefits, in which case, just using a taxable account will be all that's left.
2 years ago, I wrote a blog post The 15% solution which walks the reader through the process of optimizing their savings from a tax standpoint. The choice of investments is another matter, this simply addresses the pre-tax post-tax issue.