What is much more important vertical or horizontal bone resorption?

Vertical bone loss is much more important as it signifies progression of periodontal disease (bone loss) at a faster pace in a particular region and is conducive to regenerative treatment because of retention of bone graft. 

Horizontal bone loss signifies a chronic inflammatory form of periodontal disease, and vertical of aggressive

Both horizontal and vertical bone loss are important. Both being the result of inflammatory response towards periodontal infection. Horizontal bone loss is a little slower as far as rate of progression and activity is concerned. it can at times be stable and maintained. Vertical bone loss on the other side can progress at a faster rate but is conducive to regenerative treatment depending upon the involvement of the bone.