Thanks for your feedback Claudius. I am building up onslaught slowly now.
With my character builds (esp. 2 mastery ones) I tend to build to both mastery max's of skill point progression first (e.g 32 in each) then start to go for the skill tree afterwards.
I find that during normal difficulty you don't really need any special abilites to progress as much as you do at later levels and hence do all my skill point investment to attain 32 level in each around this time.
My Storm/Warfare guy is going well 25th level DPS 190 odd and now I will start adding to onslaught.