I did an A-B with my old band, two Mesa Powerhouse 2x10s vertically against horizontally. I was lucky enough to have a very powerful all tube amp with a small enough footprint at the time to do it safely, a Mesa Prodigy.
And the difference was pretty astounding. Nothing.
At least not the difference in clarity and projection I expected. I think like everything else, it will make a difference in some situations and not in others, there are no generic rules that can be determined by science because of the huge range of variables that will change with each room, even each song (I suspect Bill Fitzmaurice will punish me for that comment later).
There are however numerous cultural and aesthetic precedents - let's be honest here, no one wants to see a T-formation bass rig (sorry Bankai), and a lightweight 2x10 on it's side being apparently smothered by an SVT-II is not much better. Proportions folks. Big head, even bigger cab
The golden rules therefore
1. Stack your rig so that it looks good
2. Stack your rig so that it's safe and won't kill a punter (drummers and guitarists no so worried)
3. Stack your rig so that you and the band and ideally the audience can hear the bass