The 10 pound Lee casting furnace has a 3" diameter and the nozzle is about 1/2" from the front edge of the bottom rim of the crucible so the visibility of the pour is very convenient.
The 20 pound Lee casting furnace has a 4" diameter and the nozzle is about 1.3" from the front edge of the bottom rim of the crucible so the visibility of the pour is difficult to monitor at times.
During a casting session, some small bits of debris might build up in the nozzle area and the stream no longer flows straight down but might deflect in another direction. Guys may understand mis-directed flow a bit easier with age. You may find that having two casting furnaces of benefit as some like a different alloy depending upon the caliber one is casting. If you are considering casting with a ladle, the larger opening is desired.