Answer:
Assuming that all other factors are equal, (and they never are), I would prefer a 20% or more ROI investment, located in a tax deferred, or tax exempt account!
Assuming that you are for some reason entirely unmotivated to look for better investments than the two you have pointed out, why not just buy the 8.1% investment with your IRA, Roth IRA, 401(k), SEP IRA, or spousal IRA funds?
You can then pretty much ignore your tax rate. Just move over any portion of your standard deduction that remains unused near the end of the year, via a Roth conversion. You will pay no additional taxes, and the money converted will forever remain free of RMDs, (which will otherwise occur later on), for the rest of your life!
Step-by-step explanation:
Use your now freed up taxable money to invest in tax free transactions, (similar to the effect of having a periodic Roth IRA), and live happily ever after!
As always, upvote and follow me, either if you are entertained by my answers, or, if you are sufficiently curious to investigate my other answers posted on Quora, and realize that you COULD be making FAR MORE money with your investments than you have been!
Thank you for reading and I hope it helps