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
Vertical Cons:
1.I know people that have problems with them leaking (biggest con for me)
2.They will freeze and not work in very cold temperatures(read that on here)
3.If you want to clean your container, you have to do it on it's side.
4.You need to hang your container up high so the chickens can get to the nipples
Vertical Pro's
They're inexpensive (for a reason)