@Erik Sherburne I have used both tools in my investing career. In fact, I used both tools to purchase my first deal! I had to do a cash out refi and a HELOC on my Riverside home in order to have enough to purchase my four unit deal.
The advantage to the cash out refi is that you are fixing your rate. It is better if you want to hold the property long term and pay down principle. The advantage to the HELOC is that the closing costs are way less, but typically they don't have a fixed rate and the amortization schedule is a lot shorter. I would do a HELOC as long as you are going to aggressively pay it down and re-use it.