I've used an upright tank and, as said it has a smaller footprint and is fine if securely fixed, but I prefer a flatter tank as I put marine grade ply on top and use the area on top of the tank to store my stuff. I found the smaller footprint didn't help me as I don't like my stuff getting wet and the floor of my van always has some mositure on it, so I prefer flat tanks to get the most use out of the space I have.
I have Ford transit custom, but I'm a qualified pest controller so I need the space for traps, seagull spikes etc.