which is the best content holder?

iframe
div (with scroll)

which one is better for displaying content? which one is easier to set up and customize?

It's all a matter of preference. You could put an iframe into a div and control exactly where it is on the page, but I don't think older versions of Netscape support iframes too well. So you might want to look at how compatible iframes are before implementing one.

I personally would use a div (with scroll) for my content area because you could change the graphics and it's just overall more flexible.