Good question to ask, but I think it has more to do with the people than the kind of truck. My wife and I drive a straight truck. We look at van driver teams and wonder how on earth they do it. I also know of at least one van driver team that wouldn't dream of driving a straight truck. As far as I know, they love the work as much as we do. It's not so much a question of which style of expediting is best? It's which style is best for YOU?
You are asking the perfect question to help you determine the answer. There are differences between the two styles to be sure. The differences include the requirement to keep a log book (or lack of requirement), living space in the vehicle, fuel costs, parking, truck routes, truck stop benefits (from fuel purchases, big trucks burn more fuel), etc.
I know people succeed in both expediting styles. One thing that will likely keep us in a straight truck forever, and this is goofy I admit, is the view when you drive. I love sitting up high. I'd be frustrated in a van because I could not see as much. I guess I feel safer too. If I was going to be in a wreck, I'd much rather be in a truck than a van.