2-axle sideboard trailer or 4-axle hitch trailer

Although I normally run with a semi trailer this question keeps bugging me. I normally ignore the 4-axle hitch trailer since I have the feeling it's more of a drag than the 2-axle sideboard.

However I really can't back this up with facts. Before I engage in testing, has anyone went into depth with this one already?

Main consideration ofcourse when loaded, say beams or slabs.