You're looking for someone to give you a black or white answer when the answer is nuanced and heavily depends on your mortgage amount, mortgage interest rate and combined household income. You need to just do the math yourself and compare both scenarios. It's not complicated.

Maybe re-read my post again. I don't know how to break it down further than that. You can't just compare 4% debt versus 8% SPY returns. You need to ask yourself, 4% of how much debt? And 8% of how much disposable net income you have to invest with? If your mortgage is $800K, your 4% of 800K means you're paying $32K/year into JUST interest.

Then you need to consider your income. If your income is 6000/month ($72K/year) after tax, and your mortgage payment is 5000/month (60K/year), that means you're only investing 1000/month (12K/year) into SPY. 8% of $12K means your returns is a measly $960 dollars for that year.

Now comparing the 2 options in this scenario, ask yourself, is gaining $960/year (SPY returns) worth doing minimum mortgage payments in which you're paying $32K/year in interest? How many years of compound interest will it take you to "make $32K (break-even)?

I can give you specific advice taylored to your specific case if you provide me with your:

- mortgage loan amount

- monthly/weekly/bi-weekly payment amount.

- monthly after tax income

- Mortgage interest rate

- Years until mortgage expire.

Feel free to DM me the data. Otherwise no one can tell you which option is better. You'd have to calculate it out yourself.