Putting extra cash into a bigger down payment (or an extra yearly mortgage payment) buys a guaranteed outcome: less interest paid, because you're borrowing or owing less. Investing that same cash elsewhere trades that guarantee for an uncertain, typically higher, expected return. Neither is universally right — this shows the annual return the alternative investment would need to clear to beat the guaranteed interest saved, so you can compare it against what you actually expect to earn.

🇵🇹 This model doesn't account for the tax treatment of investment gains in Portugal (mais-valias at 28% for most fund/ETF and stock gains, PPR gains taxed on a more favourable sliding scale) — after-tax, the return needed elsewhere to beat overpaying or a bigger down payment is higher than the raw break-even rate shown here.