Why would I buy one over the other.
So, to address your direct question...
All of the three businesses that offer these ATB (auto torque biasing) differentials for Fuso/Isuzu trucks use the same basic Torsen style design. As to which is "best", well, that is technically difficult to say.
As has been mentioned, there are slight functionality differences between these units. My understanding is that Alan's diff has a spring loading setup which will auto engage the diff if one wheel was getting no traction at all (off the ground etc.) whereas the others will require a bit of brake pedal force to do the same thing.
Given that these are high load related devices, the quality of the material they are made from and the tolerance of the individual parts is probably key here, but unless you had all three available for testing, finding out that information would probably be challenging. Unless someone were to do a technical and/or destructive test of all three, which I am pretty sure has not been done (well, not publicly at least) it would be virtually impossible to know which is actually made better and which will perform best and last the longest.
Every manufacturer will probably say that what they sell is the best; why wouldn't they?
Sadly, the only thing that you can really do (at this time) is to base your purchasing choice on the feedback users have given with the different versions. Find out, if you can, if there have been any failures. I would also check the warranty situation with each type, so you know what will happen if there is a problem with the diff months or years down the track.