There is also the concept of your total risk across accounts to consider. When possible, it's preferable to put those investments with a higher growth potential into the Roth, and keep the traditional accounts as conservative as is reasonable to maintain the balance. In this way, the Roth accepts the higher risk, but also the potentially higher gains; while the traditional forms your 'rainy day' crash protection funds. If your traditional account(s) only average 3%, but your roth is hammering a rising market at 12%, you could still be averaging a respectable 6 or 7% across the entire portfolio.
If the market does great over the course of your career, your roth balance will likely be huge, and forever untaxable; while your traditional is likely to have grown, but not to an outstanding degree. On the flip side, if the market sucked during your career; your roth balance wouldn't be something to crow about, but your traditional is still likely to be at least as much as what you actually contributed. In the prior condition, life is good and your tax bracket in retirement is a good problem to have, but you have minimized it. In the latter, you are not likely going to be in a high bracket in any condition.