I chose vertical, because they focus more on the blow the flying shit out things and dodge the bullets aspect.
Horizontal often seem to focus on dodge-the-scenery.
however, Dead moon and Aldynes are my favorite horizontals though, but they are less-scenery dodging than others, like samalander and sinistron.
and Sapphire, Steam Hearts, and Tatsujin are my favorite verticals...
They're mostly just no holds barred shoot-everything-until-you-enter-a-fit-of-epilepsy games.
Well, Steam Hearts is more like "shoot stuff til you see tits", but,