The difference between a traditional and a Roth IRA are that in the traditional, you pay no taxes on the income now, but you do when you withdraw them. Contributions to a Roth, in contrast, are taxed now, but not when you withdraw them.
For the Roth to be the better choice, the taxes you pay now should be less than what you would pay if it were a Roth. In other words, if your income from your retirement account is going to be higher than what you currently earn, then you should choose a Roth.
Assuming that you wish to live on the proceeds of the account (rather than drawing down the principal (I know, there are RMDs to worry about as well, but to keep this simple I'll ignore them)), and that you will have an annual real return of 5%:
With 10 years to save, Roth is the better deal when you contribute 13% or more of your income.
With 15 years to save, you need to contribute about 8% or more for the Roth to be a better choice.
With 20 years to save, your minimum contribution for a Roth to make sense would be about 5% of your income.
Naturally, there are a lot of assumptions in there. For one, I didn't account for inflation or CPI increases - but I also chose a pessimistic rate of return. I'd recommend seeing a CPA or investment advisor to talk about your specific numbers.