I have a slightly different view than @Andrew Postell. I don't believe rates are going up anytime soon due to the deflationary environment we live in and will continue to be in driven by the exponential force of technology. The Fed targets inflation around 2% and what have rates done since 2009 - gone 1 direction, down. The money printing continues and this will drive rates lower. The Fed has also stated already they don't plan to raise rates through at least 2023 and in my opinion, they won't for a long time because of all the money printing going on.
All that said, the HELOC is your best option. If you can get a 1st position HELOC, and treat the HELOC as your checking account - even better. You'll pay it off in no time, especially with the additional cash flow from the rental. If you do a cash out refinance, your payment will go up more than likely.
At the end of the day, you can always refinance out of the HELOC if you want a fixed rate and get into a mortgage. The HELOC just gives you flexibility and more control of your cash.