The short profile cantis have a narrower range of acceptable straddle height and vary mechanic advantage more during their travel (unfortunately decreasing in MA as you get deeper into the lever), but have shorter arms and in most cases will feel more powerful. They also will interfere less with racks/fenders/panniers less if that's relevant, and also can be nice to have more tucked in on the rear for mounts/dismounts during cross racing. Medium (truly long is pretty much extinct) cantis are less sensitive to straddle height. Both work fine with road levers.
https://www.sheldonbrown.com/cantilever-geometry.html is a pretty good lay guide. I had an article that laid out the physics of cantis in insane/academic detail but can't find it right now.
For practical advise, try to get cantis that use threaded linear pull style pads--they're way easier to set up than older smooth post pads, even though both work totally fine in the end.
There's a lot of advise that creating a 90 degree straddle angle leads to highest MA, but really pretty much the lower the straddle wire, the higher MA in all real world scenarios.