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.
The two have their advantages when buying online most especially when the store is not reliable. Trade with coin might not be refunded if your purchase is not delivered, it is a mindset you prepare yourself for, Not that that it happens every time. Most banks offer this services if u try. My observation is that localbitcoin traders get your bitcoin into fiat at higher rate base on dollar at blackmarket in some countries. Going to market with fiat converted will make a cheaper purchase