These are all small case alternators but there is a considerable size difference between the two models so anyone in any doubt of which to select should start by looking at the dimensions. Here is the Balmar datasheet that includes dimensions but the example below might help to summarize the principal difference between them.
Surprisingly the when we look at the key dimension front to back the AT-200 alternator is shorter, see the full dimension sheet for details.
All of these alternators require high performance pulleys, either a double V-Belt or a multi ribbed belt and pulley system. There is just too much power required to be able to transfer it with a single V-Belt
All these alternators require the MC-614-H regulator to reach their full potential. The field current required to produce maximum output cannot be provided by a lesser regulator. The XT alternators do include an internal backup regulator, this is included as a "get you home" spare and not intended to be the sole regulator.
These are hard working alternators and deserve to be protected by a temperature sensor. The sensor is included in when you purchase a kit but if buying this as a spare consider a spare temperature sensor also.