Balancing question
Since nose weight is located on or near the axis of roll it has little or no affect on the roll rate. In a barrel roll rather than an axial roll the mass of the nose can be located away from the roll axis and nose weight comes into play.
The difference in roll rate between horizontal and vertical flight is related to the differences between those two modes of flight. In the vertical mode more engine power and thus more torque is needed compared to the horizontal mode. If the roll rate in vertical flight is greater in one direction than the other, then I would conclude that the difference in roll rate is torque related.
It may be that you are throttled back enough in horizontal flight that it is actually slower than full throttle vertical flight. In which case the roll rate will be greater in high speed flight because aileron response is proportional to the square of the airspeed.
In horizontal flight part of the wing's work is to produce lift and only the surplus from aileron deflection (which is limited by the stall) is available to roll the plane. In vertical flight mode, the wing is not producing net lift and all it's potential lift due to aileron deflection is available to roll the plane.
Another possibility is that the faster roll rate in vertical mode is an illusion. Have you actually timed the roll rate?