Depends partly on how much cash you have. If I were you, I’d try to keep six months worth of living expenses in cash. That way if you lose your job or the renter stops paying rent, you’ve got a cushion. After that, I’d go ahead and pay down the loan as much as possible. Like you, I can’t think of any reasonably safe investment that pays more than you’re going to pay to borrow the money. So, why pay to borrow money when you’ve got cash?
