The real question is...
... How much height is the rocket expected to add to the final altitude?
Is it is not much more than the balloon alone would reach then it is not worth too much worry about launch and control. Just getting the thing to altitude and firing is probably enough. A straight, upward thrust phase would be nice but up, down or in circles is not truly significant to the whole operation.
On the other hand if it is then then adding a proper vectored thrust rocket control is critical. It needs to be properly controlled in both thrust and glide phases and this requires two separate control systems. This obviously adds weight and and a lot of complexity. It needs to be thought of a rocket with wings more than a glider with an engine. This does mean however that if the rocket is properly able to control it's flight direction then the launch isn't so much of a dilemma. If the launch becomes the most critical part of the mission then the launch method that is least likely to fail is the most suitable. Any shortcoming will at least be partly corrected by the smarter vehicle.