There are no taxable gains in a Roth account - the money is all after-tax, with restrictions on withdrawal. In a Traditional IRA, all of the money is pre-tax, so you owe taxes on 100% of the withdrawal.
OP, can you give a "time until retirement", and "time before 60 in retirement"?
For example, if you're retiring soon and reaching 60 soon, that favors investing in taxable. Each year, you'd get ~2% dividends and probably owe 15% tax, or roughly 0.3% tax (of the total). And your gains would be modest, so the tax paid on withdrawal would also be modest. But both of those costs increase over time, so the timing matters.