If you use Bitcoin on trading sites, you could earn some Bitcoins in return. On the contrary, if you use Bitcoin on online shops or marketplace, you're just spending them. What would you choose, earning or spending? That was a trick question, of course the answer is both. So there's no point choosing over the two. Trade so you could earn more Bitcoin and spend the Bitcoin you earned on whatever you want to buy. That's what life is.
I think you can not choose 2, the choice is only 1. Depending on the situation, if in the middle of investing or trading then we transact on exchange site and when we need something goods then we will visit the online trading site.