This is a problem I have pondered as well. The answer to your question depends on whether whether you are build for the purists, or whether your prime emphasis is simple visual appeal.
Here are the scale dimensions for a hypothetical P-51 model. The 1:1 aircraft length is assumed to be 32.2 ft.
1/32 Scale - 12 1/16 in
1/35 Scale - 11 1/16 in
1/48 Scale - 8 1/16 in
It depends on how you intend to pose the aircraft with respect to the armor. If you plan to have a tank parked next to a aircraft, then the 1/32 scale would work best. Would most people notice the 1 inch difference in the aircraft length? I doubt it. For one thing, armor and aircraft are typically not found in close association, so most people don't have an innate appreciation for the 10% error between a 1/32 P-51 and a 1/35 Sherman, or an F-16 and an M1A1. If possible I would put the aircraft behind and perhaps slightly lower than the armor to visually minimize the this error.
On the other hand, if you plan to have the aircraft in flying position, such as Redleg's Spits working over the convoy, then you might be able to arrange the 1/48 scale aircraft to enhance the illusion of distance. This would work, for instance, if you had an F-16 coming from behind to attack an Iraqi T-72.