Without knowing interest rates, other investments, etc., I would be inclined to pay off the student loan, because it seems you'd be able to build back your savings pretty quickly (you'd still be left with 20K, out of the 30K you need) , and because it reduces your monthly expenses.
I would also do a little thinking on how much you are pouring into one particular property vs. purchasing another. The additional income you think you could generate is great! - but unless you live in a city where basement apartments are common, most people would place a higher value on living at or above ground, so maybe your money should go toward a different property. Even though your return on your investment is lower, your assets are better diversified. Then again, if this will be a unit used for AirBnB, then it might be easier to manage if it's the same unit.
It's all about opportunity cost.
Could you tell us more about your AirBNB, etc. business/income and how it's been going for you? It sounds like a cool idea.