pros and cons for both
flat more stable and can put things on top
upright takes up less floor space and able to get more water out of the tank when your low
I really think its down to personal preferance and the van you are fitting your set up in.
I have a 650 upright and half full it sloshes about like crazy even though supposedly "baffled", but then I had a 250 flat before that (unbaffled) equally as bad.
Some others have suggested before that you baffle yourself with lengths of perforated drainage hose inside the tank, and this is what I plan to do when I can source some.
I was going to order a custom tank with proper sectional baffling but I've blown the cash on a holiday instead
