Pay off student loan debt or invest?

Is there a general rule of thumb on student loan interest rates and whether or not it's better to pay off ASAP vs invest in an index fund? Sold a lot of company stock from an ESPP and RSU program that happens to be the value of our household's student loan debt that is just entering repayment after graduation. Can't tell if a 5 or 6% is worth drawing out or paying off in one go. Not worried about rainy day or emergency fund and already maxing out my retirement. So really it's a question of debt payoff or non retirement investment.