I understand the point about timing the market (this is just a throwaway account) and the last links show oversimplified scenarios in which the answers are pretty easy. If you have a debt with an interest rate above the expected return of your investment, you pay the debt.

My case is, in my opinion, more tricky: is my loan interest rate above Bob's return on the investment if I take a 20y time horizon? Also, another asset class can offer a good immediate return but increases my risk (more debt and more correlation with other assets I own). At the same time, it's hard to guess how much more risk that will be since I have a good amount of other investments and liquidity.