raph55 Personal preference mostly. The 9% cast speed makes the gameplay feel a bit smoother.
As this was a beta build, I'll be updating the skill trees for this guide later when I get to endgame and can play around with the updated/patched skills. The rest of the choices should stay mostly intact.
Glad you're enjoying it! I had a blast (pun intended) with this in beta, and with the huge buff to Aergon's Fireball to bring it in line with other invocations that we got with v1.0 I expect this to do even better. All runemasters are going to lose some survivability due to the changes to wards, but it should still play the same.