Non deductible IRA or brokerage account?

We are not eligible for a Roth IRA. Additionally we would be subject to the pro rata rule for the back door Roth due to existing IRAs. Should we contribute to a non deductible IRA? Or just contribute to a brokerage account?
We were thinking with the non deductible IRA, we would be taxed at ordinary income on earnings upon retirement whereas a brokerage account would be capital gains rate which is lower currently. Thoughts?