The things to compare are

1. Expected marginal tax rate now vs. Expected effective tax rate in retirement.

2. Number of years until retirement where your contributions will compound.

For 1, Cheddar Stacker already mentioned that a good rule of thumb is the 25% marginal rate as the cutoff point towards Traditional accounts because *most* people here will have an expected effective tax rate in retirement significantly lower than 25% (<8% is common).

For 2, the longer your money has to compound, the more valuable Roth's are. For example, if your money compounds at 8% (i.e. doubles ever 9-ish years) and you have 36 years until retirement your Roth will have 16 times as much money at the end and the only taxes paid will be the initial 25% (or whatever the marginal rate is) on the initial contribution, the other 94% won't be taxed at all. This works out to an effective tax rate of 1.56% on the final balance. If you only have 9 years until retirement though, you'll double once, end up paying 25% taxes on half the money and end up with an effective tax rate of 12.5% on the final balance.

So the longer you won't touch the money, the better a Roth is. The greater the difference between your current marginal tax rate and your final effective rate, the better a traditional is. The only way to know for sure for your situation is to do the math.

You've got #1 exactly right but #2 is questionable.

The correct way to figure this out is exactly as you said. Compare your current MARGINAL tax rate to your expected future EFFECTIVE tax rate. This is a key point of confusion for most people (including myself starting out). Look at your 2013 taxes. In my case, my marginal rate was 25% and my effective rate (federal income tax paid / total wages) was around 17%.

So even if my income and spending is exactly the same in retirement (which it won't be due to paid off house and no work related clothing etc.) I would save 8% in taxes by going with the traditional 401(k).

On point #2 you need to compare apples to apples. That means that you count the extra taxes you pay on the Roth as added contributions for a traditional 401(k). So say you have $10,000 pre-tax to put in your 401(k). Two scenarios:

Traditional: You pay no income tax now. $10,000 goes in. Let's say at retirement it is worth 5x more or $50,000. Then you take it out at a 17% effective rate for an after tax amount of $41,500.

Roth: You pay 25% on the money now since the 401(k) is off the top of your earnings. $7,500 goes in. x5 it is $37,500. You pay no taxes taking it out.

The difference is $4,000. Or 8% on $50,000.

The same thing works whether it is 5x or 50x. In all cases the difference between marginal and effective rates means you keep 8% rather than giving it to Uncle Sam.