Our old van had the Manchester "van style" horizontal tank, with the fill valve tilted down 45 degrees to be accessible from under the van. It also had a rock guard that hung below that. This tank has to hang lower to get access to fill, and it got scratched up and rusty from gravel. The stone guard also got bent up.
The Sportsmobile tank is the same size, but the fill valve is horizontal and is accessed through an access door on the side of the van. It has a rock guard which is small since it sits above the bottom of the rocker panel. The entire tank sits higher and it seems to get less damage from rocks etc.
We selected the SMB tank for our current van to keep the most ground clearance. Realistically there is not a huge difference, and either one works fine. They will both need to be sandblasted and repainted at some point.
On a side note the SMB hinged access cover also hides the electrical power inlet.