I want to first say that it is absolutely possible that my mental (and otherwise) modelling might be way off base, but my next statement is that the math does not necessarily support that, assuming you're saving at a rate that will maintain a similar standard of living as your pre-retirement years.
Let's assume first that you are saving every last penny you can afford. If so, then saving via Roth is by definition going to allow you to save less. (Every dollar you pay in tax now is a dollar you cannot save.) If your top marginal rate is in the 25% tax bracket, that means you're saving potentially 25% less dollars, and more as you go through the brackets. As earnings compound, your total is still that same percentage less at whatever point in time you wish to look, assuming you have reasonably similar funds to select from. The question then comes back to what taxes did you avoid in order to save traditionally, and what taxes will you have to pay when you start to make withdrawals. If you saved 25 or 28 percent, and you end up paying 20, then the math is clearly on the side of the traditional 401K.
Please, tell me where I'm wrong. I'm absolutely interested.
I guess a lot of it depends on what you predict tax rates will be in the future. Even if you're in the 25% tax bracket currently, that's only about $1,833 you're paying in taxes on $7,333 earned to have $5,500 to put in the Roth. Using a conservative 8% return rate, it will be worth $119,484 in 40 years. To withdraw it will cost no additional taxes.
Now, let's use that $7,333 pre-tax number and say it was invested in a 401k/Traditional IRA instead. It's worth $159,305 in 40 years. If you're taxed at 20% on that, you'd come out ahead of the Roth. If you're taxed at 25%, the Roth gets a very tiny advantage. If you're taxed at 30%, then the Roth wins. Obviously it's all a guessing game as to what specific tax rates will be in the future, but it's likely they'll be going up, not down.
It also depends on how you plan to invest in the future. The $5,500 yearly max for a Roth will probably go up over time, but it will surely stay far behind the 401k yearly max. For estate planning purposes, I'd rather have a larger portion of my wealth in a Roth, and the only way I can do that is by maxing it out every year. It's easier to play catch up with the 401k later, given the higher limits (and the possibility/likelihood of jumping into the next higher tax bracket, thus giving the 401k a bigger advantage later, due to the current tax savings being greater). So for that reason, I like 401k to employer match first, followed by Roth, followed by 401k max.
Let's also throw into the equation how you intend to live in retirement. Will you be looking to spend your retirement savings, or are you mostly going to sit on your money? If you're spending, then the required minimum distributions from a 401k/Traditional IRA aren't a problem. If you're just going to sit on your money, then they are a problem, because you don't have another way to invest those distributions in a tax-advantaged account if you aren't working. So while a Roth owner can just sit on his money and continue to let it grow tax free, an individual who is forced to take disbursements from a 401k/Traditional IRA will have to pay taxes on those disbursements, plus will pay capital gains tax on any future growth if those disbursements are reinvested.
I think it's a very interesting debate, and I'm not saying you're wrong to prioritize a 401k over a Roth, but for me personally, I think the Roth is a better option after you've done a 401k up to the employer match. But again, everyone's personal circumstances are going to be different, and that's the most important factor that will be guiding which to choose (ideally, of course, you don't have to choose if you max out both!).