At one point I had debt (a loan) and money available each month that could go toward one of two things: paying that debt down faster, or investing it. It's a classic dilemma, and the answer I settled on wasn't "always the same thing" — it was "it depends on the interest rate."
The rule I use: compare interest rates
The logic is simple, even if it's easy to forget day to day: if your debt's interest rate is higher than the return you expect from investing, pay off the debt first. If your debt's interest rate is low and you expect the investment to return more over the long run, investing usually wins.
For example: a credit card or consumer loan at 15-20% interest is mathematically almost impossible to "beat" by investing. There's no debate there — pay it off first, as fast as possible. A low-rate mortgage, or a loan with reasonable terms, changes the equation quite a bit though.
How I calculated it in my own case
I didn't decide by feel. I used a pay off debt or invest calculator to compare, with real numbers, what I'd end up with in each scenario after X years: paying off debt early versus investing that same amount every month. Seeing both numbers side by side takes a lot of the emotion out of the decision.
And the result isn't always what you'd expect. Sometimes the difference is small, and that's where another factor comes in that calculators don't fully capture: peace of mind.
The part that isn't just math
Carrying debt creates a psychological weight that doesn't show up in any formula. For me, there's a point where I chose to pay down debt a bit faster than the strictly optimal number, simply because I slept better knowing that balance was dropping faster.
I don't think that's irrational. I think it's recognizing that money isn't only a math problem — it's also about how you feel about your financial situation. If a debt is keeping you up at night, paying it off sooner has value that doesn't show up in a spreadsheet.
My actual approach
With expensive debt (credit cards, high-rate personal loans): I pay it off first, no exceptions. With cheap debt (a mortgage at a good rate): I keep my monthly DCA investing going and don't obsess over paying it down early, unless I have a one-off surplus.
There's no universal answer that works for everyone, but there is a right way to frame the question: compare the real interest rates, run both scenarios with actual numbers, and then decide how much weight you give to peace of mind versus the purely mathematical result.