neither of them! local store owners are small businessman. Their business run on hard cash where no exchanger fees are applicable. Whenever a small business man accepts bitcoin, they will have to again pay fees to the exchanger to exchange that particular bitcoin in to fiat.
such a store would only accept cash. but many also take credit cards. why not bitcoin
However, big corporate are easier to convince. But they run on numbers. so the dynamic price of bitcoin is not always good for their business. They will have to have a contingency plan in case the price depreciates. It is always better and easier for them to accept and spend in fiat.
big cooperations might be harder since you might have to convince more than one person.
i do not think it will always be easier and better to accept fiat, but right now their focus is on fiat. this could change.