Curling hair with curling iron or straightener?

I'm going to buy a new curling iron or straightener today, but I don't know which I should buy. I want one so I can make big, natural, loose curls. Should I use a large curling iron, or a small straightener? My hair is naturally REALLY thick and doesn't usually curl easily, or stay in long after I brush it or something. So, if you have thick hair that doesn't curl easily, what works best for you? Thanks!