All the monitors have the same size picture tube - 19". The Amplifone was made in a 25" version, but they were only used in Star Wars cockpits (AFAIK) and they are very rare.
The Amplifone is physically larger since it uses a 90 degree picture tube. It also does not have a traditional "frame" - it's just a picture tube mounted into the cabinet, with board guides on the sides of the cab to hold the deflection and HV boards. It was only used on a couple of games - most notably Star Wars. Other games were shipped with it - but also shipped with the 6100.
The Amplifone has better picture quality. Most versions of the Amplifone used a medium resolution picture tube, for a sharper picture. Also, the Amplifone is faster - the slew rate is better meaning it can draw more vectors on the screen with less flicker.
But, since the Amplifone has a different deflection angle than the WG6100 (90 degrees versus 100degrees), you'll get a slightly bowed screen when you use a 6100 on Star Wars, or an Amp on a game meant for a 6100. It's not that big of a deal, but it really bothers some people.
The Amplifone is hard to get because it had a VERY high failure rate - almost every one of the original flyback transformers failed. As such, most of them were trashed years ago. Star Wars is very commonly found with a WG6100 replacing the Amplifone for this reason. Fortunately, now replacement flybacks are available - so an Amplifone can be easily repaired. It's definitely the superior monitor, but again, hard to get. Also, since the Amplifone has no frame - you'll have to mount it in one or make something to hold the deflection and HV boards if you install it in a cabinet that didn't ship with one.
The 6100 is a pretty good monitor, relatively easy to find, easy to work on, and all the parts are available to repair it. It will electrically work in any Atari vector game. It's not quite as nice as the Amplifone - but overall I think it'll be easier to use across multiple boards/cabinets/setups. It has the same mounting ears that color raster monitor have, and will drop into any normal 19" cabinet. You will need the monitor, and an Atari power block from the bottom of the cabinet - since vector monitors use the center tapped winding for power - not a standard isolation transformer.
-Ian