On your question of what mortgage you should overpay -- your primary residence or your investment property -- our recommendation is that you should overpay your primary residence. While from a purely financial standpoint it makes sense to overpay the mortgage with the higher interest rate, from a peace of mind and risk management standpoint we always recommend that borrowers overpay the mortgage on their primary residence before overpaying other loans.
Although there are tax considerations that may support overpaying the investment property mortgage, borrowers rarely go wrong when they pay down the mortgage on their home. The sooner you pay off the mortgage on the home you live in, the sooner you can start making that payment to yourself. Plus, paying down the mortgage on your primary residence should enable you to better manage unforeseen financial challenges in the future.
Paying more than your required monthly payment is also called mortgage acceleration. We provide a thorough overview of How Mortgage Acceleration Works on FREEandCLEAR and you can use our Mortgage Acceleration Calculator to evaluate different acceleration scenarios for your loans.