Taking all your assumptions:
With Roth, you take $6112 from work, (let's call you tax rate 10%) pay $612 in taxes, and contribute $5500 (the max if you are younger than 50). This $5500 will grow to $21,283 in 20 years at 7% annual growth ($5500*(1.07^20)), and you will pay no additional taxes on it.
With the traditional IRA, you take $6112 from work, pay $612 in taxes, and contribute $5500. You will receive a tax deduction at tax time of $612 for the contribution. This money will also grow to $21,283. This will be taxed at your ordinary income rate (which we're calling 10%), costing you $2123 at the time of withdrawal. You will have $19,155 left over.
- Roth: $6112 -> $21,283
- Traditional: $6112 -> $19,155 plus your $612 tax deduction
If you invest your tax savings from every contribution to the Traditional IRA, then the numbers wash out. Perhaps a pivotal question is whether you believe you will have greater taxable earnings from your investments in retirement than you have in taxable earnings today -- affecting the rate at which you are taxed.
Even if you're paying a lot of taxes now, you're talking marginal dollars when you look at current contribution, and average tax rate when making withdrawals. IE, if you currently pay 28% on your last dollar (and assuming your contribution is entirely in your marginal rate), then you're paying 28% on all of the Roth contributions, but probably paying a lower average tax rate, due to the lower tax rates on the first many dollars.
Look at the overall average tax rate of your expected retirement income - if you're expecting to pull out $100k a year, you're probably paying less than 20% in average taxes, because the first third or so is taxed at a very low rate (0 or 15%), assuming things don't change in our tax code. Comparing that to your 28% and you have a net gain of 8% by paying the taxes later - nothing to shake a stick at.
At minimum, have enough in your traditional IRA to max out the zero tax bucket (at least $12k). Realistically you probably should have enough to max out the 15% bucket, as you presumably are well above that bucket now. Any Roth savings will be more than eliminated by this difference: 28% tax now, 15% tax later? Yes please.
A diversified combination is usually best for those expecting to have a lot of retirement savings - enough in Traditional to get at least $35k or so a year out, say, and then enough in Roth to keep your comfortable lifestyle after that.
The one caveat here is in the case when you max out your contribution levels, you may gain by using money that is not in your IRA to pay the taxes on the conversion. Talk to your tax professional or accountant to verify this will be helpful in your particular instance.
To answer your question point by point -
- This is a given. If you don't have the money to pay the tax, stay clear.
- Years to retirement is irrelevant. 20 may be a mistake, 2 may be right.
- Rate of return also irrelevant so long as the cumulative return is positive.
I'd focus on the last point. The back of my business card -
Let's focus on Single. The standard deduction and exemption add to over $10K. I look at this as "I can have $250K in my IRA, and my $10K (4%) annual withdrawal will be tax free. It takes another $36,900 to fill the 10 and 15% brackets. $922K saved pretax to have that withdrawn each year, or $1.17M total.
That said, I think that depositing to Roth in any year that one is in the 15% bracket or lower can make sense. I also like the Roth Roulette concept, if only for the fact that I am Google's first search result for that phrase.
Roth Roulette is systematically converting and recharacterizing each year the portion of the converted assets that have fallen or not risen as far in relative terms. A quick example. You own 3 volatile stocks, and convert them to 3 Roth accounts. A year later, they are (a) down 20%, (b) up 10%, (c) up 50%. You recharacterize the first two, but keep the 3rd in the Roth. You have a tax bill on say $10K, but have $15K in that Roth.