There are so many things that can be contributing to the two airplanes behaving differently. Radio setup could be one. Some TX have the capability to have reverse expo but at a glance have the same numbers so is an easy mistake. Different length servo arms, different thrust angles, lots of things at different degrees. I would agree that CG could be playing a part here and disagree that incedence has no part of it. If the stab and wing are both at zero and the CG is slightly aft it can cause a pitch instability that is more severe then if just one or the other is present. The reason is that the wing is " Hunting ". The cure would be to move the CG forward and/or add a tad of positive incedence to the wing.