Pro's for 6 semi's:
Looks better, slightly better tire wear.
Con's for 6 semi's:
Costs more, a lot heavier which means poorer acceleration-braking-and mileage, may require more modification to run depending on your truck/rear which also adds to the cost.
On my truck u can see the inners when looking at it from the sides as well, about the only way you couldn't would b if it was layin'.