Horizontal No cons about them.
Pro's
1.They work great
2.You can set the bucket on a couple cinder blocks, you don't need to hang it up high so your chickens can get to the nipples.
3.You can set the bucket down and not worry about damaging the nipples, when cleaning.
4.If you put a bird bath heater inside the bucket, the nipples won't freeze
Thanks Didn't know bird bath heaters exist. I'm planning on a horizontal one eventually. Trying to come up with a winter solution to frozen water with no electric to the coop. So have been hunting for battery aquarium heaters, will also start looking for bird bath ones.