Hierarchies and tags are not an "xor" proposition.
I always use both at the same time.
Hierarchy gives you a way to organize things according to each others. I want hierarchy, because if I deal with thing A, I may have then to specifically have a look to child B or C.
It helps with context and granularity.
While tags allow you to attach multiple categories to things and filter according to that.
You can use a hierarchy as a poor man tag, but really, you should use both.
I always use both at the same time.
Hierarchy gives you a way to organize things according to each others. I want hierarchy, because if I deal with thing A, I may have then to specifically have a look to child B or C.
It helps with context and granularity.
While tags allow you to attach multiple categories to things and filter according to that.
You can use a hierarchy as a poor man tag, but really, you should use both.