I see enough links/quotes to reliable sites that IMHO whoever wants to know the difference between ASME tank and DOT cylinder - should get it by now.
Those who prefer to use slang - it is their choice but they risk constant misunderstanding.
Coming to horizontal v/s vertical cylinders - done well both should offer the same % of propane to air bubble. Since that is forced by bleeding pipe and OPD valve - you will find some variations.
Horizontal tanks can offer slight advantage in low temperatures. Propane needs to absorb heat to vaporize and those using them in winter might experience tank freeze. At low propane level horizontal tank offers more contact surface.
I read several reports from members camping at -20F and they did not report tank freeze. I guess the build-in compartment offers enough heat coming from inside so propane freeze never happens regardless shape of cylinder.