Hi Dave, both models are round as is my current model. I heard from several guys that a round one was the most stable for open ocean use, i.e. beach launching and fishing laguna as opposed to calm water like a bay. The open or U-shaped ones I was told weren't stable enough except on calmest days. I can see that with the older style U-tubes held by open by a flimsy strap but there's some new open tubes with a removable rigid bar across the front that look good. Don't know much about those though.