Depends on what you want it to look like. A wood stand usually has a nicer look to it. I don't think that either would really be better. If you will see the stand then you might want a wooden one so that if it is finished. If you won't see it then the iron will work just as well. I think that it is up to you, as long as they are both built to hold the tank, then either one.
Powdercoat it ofcorse... If you want another tip if the sump is going in to the cellar, then throw all tubes to the rear of the tank and create a revovalble barrier so you can use the front for storage closets or book shelves. You can make it appear as if its floating ot put doors to hide whatever is inside. I did doors myself, 5 doors that open so I can store things in there.