Verticals usually save some space as others have said, also, they draft vertically obviously. They also typically have one door, opening it could stoke your fire up some or drop heat temporarily, just be in and out fast. Also they drip on one another and possibly straight on your fire (can be good and bad). Also less fuel management for insulated.
Horizontals can be temp stable, you can also make hot spots, which can be used advantageously. Also, they draft horizontal and stronger IMO. Opening a door, to me, has less effect on the fire. Some have 2 doors which can hold heat while you open up. Usually meat is also accessible without having to slide shelves out like a vertical is sometimes. They also feel less cramped. You have to feed them though...
It all depends on exact pit to pit comparisons really...