It makes sense to pay it off in your case.

You are not getting much tax benefit anyway for now. $300 per month interest expense is nothing. Your interest rate is about 5% which is not that great.

However, please make sure:

1) leave enough cash in hand in case of emergency. I would say, 6-month living expense minimum.

2) refinance your primary residence first before you apply for HELOC for your investment property.

3) Do not spend your HELOC money just for sake of getting tax benefit. Control your spending. Invest, not to spend. Buying a new car or do home improvement are spending, not investing.

4) If you do not have a better way to invest, do not take money out of your HELOC.Leave HELOC there just for next investment opportunity to pop up, or for emergency.