Steel piers win hands down, both in rigidity and move-ability when you inevitably up sticks.
When we moved from the UK, I brought my pier with us! It's a 10" thickwalled pier with 1/2" steel plates top and bottom. I had to leave my 3ft x 3ft x 3ft concrete footing for it buried in the ground though. I can imagine the new owners striking it with a shovel one day, and thinking "what's this?" - exposing a bit of it and giving it a pull
I think the reason concrete piers are winning in the poll is they're easier to come by/make, and cheaper. Not better...
Regards,
Rob.