I don't think the G'Vrahn is necessarily better than the Bin'Tak. If you get into the sides or rear of a G'Vrahn, it's...going to get hurt. It has excellent forward power, but very little in the sides and practically nothing in the rear to cover it.
Now the Bin'Tak just has guns everywhere, at least two sets. Particle arrays AND Pulse Cannons, plus a rear boresight.
The advantage the G'Vrahn has over the Bin'Tak is obvious; Speed and agility. two 45's give it that crucial Boresight ability and a fast turning radius, whereas the Bin'tak is...a dreadnought that lumbers along with sheer death.
If anything, the G'Vrahn kind of reminds me of an upgraded Var'Nic in practice; Fast, maneuverable, and able to singularly cut targets out of space, but extremely weak in other arcs. Not that 10AD of Pulse cannons is bad.