I strongly recommend getting as close to 100 mobility as possible regardless of element or subtree.
Each specific subtree has a specific reward system for doing something specific, focusing around accuracy of your attacks.
An exotic honorable mention for this element is Dragon's Shadow, which can reload your weapons when you dodge, meaning you can run melee dodge for the powerful melees in this element. Sometime I found this more useful than Ophidia Sphate (see Middle Tree).