I love trying to predict the future....
A few crazy variables:
-if you are in a high tax state now are you staying there? If you move to a low tax state you are probably better off with traditional 401k to avoid that taxes completely.
-taxes are really low right now. No matter what republicans try to tell you, historically and globally current US tax rates are really low - if you want proof check out the debt. They will likely go up at some point.
-401k deductions come under attack often. Obama considered lowering the cap and the recent republican tax plan considered lowering the cap. The reality is simple, people that can afford to save $18,000 per year don't need tax breaks relative to people who can't afford health care. If they dropped the limit to closer to $5,000 a very small percentage of the population would even notice.
-there is a limit to value on a 401k, if it's more than $1,000,000 (today's dollars) you'll end up being in the higher tax brackets when you retire anyhow, might as well pay tax now to have more flexibility oF ROTH. It sounds like a lot, but if you start young (and combine a working spouse) it's an easy target.
So there's a few reasons to go either direction. Load up 401k while you can, but load up ROTH while taxes are low. Deciding what is more likely will make you go mad.
How would I possibly know what tax bracket I'll be in when I retire? (25 years from now, probably... I don't want to retire particularly early but I DO want to move to part time work in the next 10-15 years... I like what I do but I'd LOVE it if I only did it 20 hours a week instead of 40!). This predicting-the-future thing is mysterious to me!
You throw reasonable numbers at it. Like if you worked half time you'd make half as much money. Use tax brackets as they are now and review as things change.