You aren't likely to predict your exact marginal withdrawal rate, but you can do at least a ballpark estimate. Then use that to guide your traditional vs. Roth decision.

Note the possibility of self-defeating predictions: predict high taxable income > contribute to Roth > get low taxable income; predict low taxable income > contribute to traditional > get high taxable income

Here is one procedure to consider:

1) Include any pension amount (or continued income if planned retirement date is ~never) that you can't defer in return for higher payments when you do start

2) Take current traditional balance and predict value at retirement (e.g., with Excel's FV function) using a conservative real return, maybe 3% or so. Take 4% of that value as an annual withdrawal.

3) Take current taxable balance and predict value at retirement (e.g., with Excel's FV function) using a conservative real return, maybe 3% or so. Take 2% of that value as qualified dividends.

4a) Decide whether SS income should be considered, or whether you will be able to do enough traditional->Roth conversions before taking SS.

4b) Include SS income projections (using today's dollars) if needed from step 4a.

5) Calculate marginal rate using today's tax law on the numbers from step 1-4.

6) Make your

traditional vs. Roth decision for this year's contribution

7) Repeat steps 1-6 every year until retirement

For 2017 I'd guess the answer will be "traditional" - but instead of guessing, what do you get if you run the numbers?