Yeah it definitely varies pretty heavily on the atomizer. Some airflows are just designed to work better with vertical coils, and sometimes it's easier/more efficient to set up the wick in a vertical way if the juice well is particularly deep.
I really didn't like the Subtank's horizontal coils, or the Nautilus horizontal coils, but once they switched to verticals I noticed a pretty drastic improvement in lifespan, flavor, and overall quality. They also changed the wicking material during this time, so that could have something to do with it too.
So in summation, all my RDA's are horizontal dual coils (I have one single coil clapton too), and all my pre-built heads are vertical coils. I actually still have a box of the SubTank horizontal heads that are probably never going to get used.