think a case of a user with diff-tpl in a folder with a diff-tpl.
what should be the resulting template in your point of view?
should only one apply? which one?
or should both apply? which first?
First of all, I have to say... I maybe wrong, but I can't see two seperate diff templates (a user diff-tpl & a folder diff-tpl in the same folder) working properly. What would be the resulting output? They might conflict in some way. I'm still not sure what the true benefit of this would be. Ok I got that out of my system.

Hmmm. I'm now thinking of my existing diff templates. Some serve multiple users and others serve all users. In the scenerio you suggest, I think the folder diff-tpl should have priority as it serves more than just one user and can also include global changes the admin wants to his site.
If a user diff-tpl was created, there was a reason for it. So I guess both user and folder diff-tpl should apply. You asked the question - which first? That question suggests to me that you are thinking of displaying 2 diff templates (user and folder). Somehow I think that would confuse the viewer and might not give the desired output the admin wanted. My answer to "which first" is I'm not sure. I think the best solution would be to combine the information from the user diff-tpl into the folder diff-tpl (1 resulting folder+user diff-tpl) which would still give the folder diff-tpl priority but also include the user diff-tpl information.
I'm going to have to think some more about this.