If your main goal is to play gunbreaker I would stop the msq as you have thought about, and pick up with the gunbreaker. There are a few reasons to do this. 1. They will likely add gear to the 4.1-4.5 msq so that you can get the gear needed to complete the SB msq. if you were to do it the other way you'd need to aquire gear for the new job either now through tomestones crafting ect. It would also take some of the grind away, since you will have msq quest to do and if you preordered, the new ear rings that give bonus xp to level 70.