I don't know why horizontal nipples would be less likely to freeze than vertical ones. In fact I would guess they would be more likely to do so since the water drips onto the plastic part where it can freeze. But I've never used them so I have no personal experience/knowledge.
The way the horizontals are designed the sealing point is inside the water, and they generally leak less... Yes, the small cupped area (holds about a single drop) outside can freeze but that is generally knocked away with the first peck, and then since the rest of the nipple mechanism is inside the heated water they flow again... Sometimes they will build up ice and freeze one the outside, but if the chickens don't knock it free you can or some just hit it with a hair dryer... In truth they do resist freezing better...