ROI also known as return of investment. And is important have positive ROI than 0% because that means that you are in positive numbers and you get more revenue than your investments. Is interesting that you could have only -100% ROI (which mean that you lost all of your investment) or you could have high positive ROI like 1 000% (which means that you get a lot of money).
The calculation of ROAS is simple:
ROI = ((revenue - costs) / cost * 100)
Is better use ROI or ROAS?
I think that isn’t important to thinking if is better ROI or ROAS. Is important know how are both metrics calculated. ROI is about revenue and costs by default and ROAS is about revenue and adSpends so that means that ROI is probably more interesting for business owners which needs to earn money because it shows all costs.