If you had dual aileron servos, and dual elevator servos, as some high-end 3D planes have, you would not be able to trim out each of the servos individually; you would use a Y-harness, and mechanically adjust the control surfaces. 6 channel radios have 2 'p-mixes' where you can program in two separate mixes (linking a slave to a master servo is a mix).
Also llinking rudder to aileron is a (pre programmed) mix. The 7ch has 3 p-mixes, as well as a snap-roll function, and triple rates instead of dual rates. Both have common mixes already installed, in addition to the user set p-mixes, such as flaperon, aileron to rudder, and an air brake function.

I'm on my (third?) plane, and just starting to use dual rates and expo, and probably a little prematurely. 6ch will do you good for a long time.