it depends. if I can buy something with fiat, I'll use fiat. however, if the shop selling the thing I want to buy is far, let's say it's abroad, then I will use bitcoin as long as the shop accept bitcoin. because bitcoin is more profitable as investment.
Why do you say that it is more profitable as an investment but you use it to buy things for it. Then you should just use fiat always than spending you bitcoins because as you said it is a more profitable as an investment. This is what I believe too. There is little point in using it, then regret using it because you realize the prices are high by the time you already paid for something using bitcoins.