For our 30 to 35 chickens, we use two heat bases with our metal waterers, one is 2 gal., the other a 3 gal. size. It's been difficult to find the 3 gal. metal waterers, my preferred size, so had to buy a smaller one recently.
The 5 gal metal waterers are HEAVY, and can only be lifter by the larger handle 'inside' the outer metal sleeve. the little handle on the top of the sleeve will fail with 5 gallons of water inside. Our other choice is to get the bigger unit, and fill it with 3 gallons max.
Haven't liked the plastic heater waterers, they are a pain to fill upside down. Ours is an emergency backup, stored.
About the heater bases, they are warmest in the center, not around the edges. Not a problem!
Mary