Gnome Launcher easily has the highest DPS, although, you can mix them up so either is fine but Gnome usually comes out with the most DPS by far.
No, it is not better than BOTT. Brain Staff may have higher damage but you must be in close encounter for that damage which is not ideal if you are a mage. BOTT is a very good weapon for mages so if you are one I strongly suggest keeping it or perhaps buying it.
That highly depends on your progress on the game. If you're midtier and have decent gear I suggest staying that way for a bit as ranged in my opinion, only is good later on in the game once you get: Gunfists or, Stargun added up with the Forgotten life set.