Just some additional thoughts:
A. Generally, in terms of using funds, Roth money should be withdrawn/used last. Also, any Roth contributions can always be taken out without taxes or penalties. So, I am not sure it makes much of a practical difference which of the two Roth accounts gets funded. The 59.5 age limit might be the tie-breaker as RichMoose suggests, but it might not make much of a difference. My understanding:
You will not be subject to the 10% TAX PENALTY on withdrawals of any CONTRIBUTIONS. You will not be subject to the 10% TAX PENALTY on withdrawals of any conversions or rollovers if you meet a 5-year test on the specific conversion/rollover or if you are over 59.5 (or other qualifying circumstance). You will not be subject to the 10% TAX PENALTY on withdrawal of EARNINGS if you are over 59.5 (or have another qualifying circumstance).
Contributions and Rollover/Conversion amounts in a Roth are not subject to income tax (they were already paid, for example when the conversion took place). Earnings will not be subject to income tax if you are over 59.5 (or have another qualifying circumstance) and have had ANY Roth open at least 5-years (i.e., meeting the 5-year test).
Memo: If you are under 59.5, EARNINGS are not subject to the 10% TAX PENALTY or INCOME TAX if used under a specific qualifying circumstance (e.g., under a higher education exception or under the $10 k first-time home buyer exception).
Your earnings are withdrawn last, after withdrawal of funds from contributions, rollovers, and conversions.
B. The 401k should be funded at least to get any match. Assuming that additional 401k contributions are BEFORE TAX, then it may then come down to tax rates. Note that Required Minimum Distributions may cause higher tax rates, so should be reflected in the future tax rate assumptions. Also, a surviving spouse may be in a higher single bracket.
Say, I have $1000 that I could now put in the 401k.
1. Assume my tax rate now is 24% and my tax rate later is 12%.
Case 1-a) I put the $1000 in the 401k, and it doubles over time. Then I have $2000 later. After 12% tax, I get $1760.
Case 1- b) I instead decide to put the $1000 after 24% tax equivalent of $760 into a Roth and it doubles over time. Then I have $1520 later after no further tax.
So, in this tax rate scenario (lower rates later), it is better to fund the 401k.
2. Assume my tax rate now is 12% and my tax rate later is 24%.
Case 2-a) I put the $1000 in the 401k, and it doubles over time. Then I have $2000 later. After 24% tax, I get $1520.
Case 2- b) I instead decide to put the $1000 after 12% tax equivalent of $880 into a Roth and it doubles over time. Then I have $1760 after no further tax.
So, in this tax rate scenario (higher rates later), it is better to fund the Roth.
3. Assume my tax rate now is 24% and my tax rate later is the same 24%.
Case 3-a) I put the $1000 in the 401k, and it doubles over time. Then I have $2000 later. After 24% tax, I get $1520.
Case 3- b) I instead decide to put the $1000 after 24% tax equivalent of $760 into a Roth and it doubles over time. Then I have $1520 after no further tax.
So, in this tax rate scenario (same rates later), it makes no difference whether to fund the 401k or Roth.
However, for break-even, I probably would be inclined to go with the 401k and avoid the sure tax cost now.