Thank you. Very helpful.
And I'm gathering that you did use the horizontals during freezing weather and they stayed thawed with your heaters in the bucket from what you wrote.
Yes, that is true. On other threads I (and others) have written quote a bit about keeping nipples thawed. For vertical nipples, only an aquarium heater worked well and it worked down to -9F, likely less with a bit of wind chill. I used 100W submersible (non-shattering, supposedly) aquarium heater but I have heard others using as little as 25W. I didn't want to take a chance since it can get quite cold here. The heater's lowest setting is about 60F, so the water was always quite warm, probably much warmer than it needed to be, but at least it didn't freeze. Next winter I will probably somehow insulate the bucket to conserve electricity.
For the horizontal nipples, I used a 250W stock tank de-icer. IIRC, it comes on at 35F and shuts off at 45F, so even though it's 250W, it takes less time to heat the water and doesn't heat it as much. It worked liked a charm with the horizontal nipples, though I wasn't able to test the setup below about 5F. Because of the design of the h-nipples, I expect (and have heard) they are naturally much less prone to freezing up than the vertical ones.
Both heaters are supposed to have auto shut-off safety features in the case that the bucket they are in runs dry. With both heaters, I had the bucket run low, low enough that a portion of the heater was exposed to air. Both of them, when discovered, steamed and sizzled. The aquarium heater also got hot enough to melt one of the rubber suction cups that secure it to the bottom of the bucket. Perhaps it hadn't reached the point to get hot enough to trip the safety auto shut-off, but I will certainly keep a better eye on water levels this winter.
I think I paid about $30 each for the aquarium heater and stock tank de-icer.