Amount of model memory.
Number of available mixes.
Types of mixes.

Remember all of the 4+ channel radios have the basic functions. Flaperons spoilerons, ect are a mix
to the aileron function. You need a radio with more channels and available mixes to operate these

Flaps Retracts, bomb drops. are a completely seperate channel with seperate servos. Again set to a switch
More channels needed.

There is alot more to todays computer radios than just the 4 basic channels. take my Friends pattern plane for example
when he pulls vertical he hits a switch which adds right rudder as a direct result to adding throttle. this helps keep the plane
from veering off line . Again this is a mix using two of the basic funtions programmed together.

I don't know waht type of aircraft you see yourself flying in the future. I picked pattern flying as my type so I need a good radio
with quite a few open mixes. but I can't see ever needing more than a 9 channel radio for myself.

You also said you wanted the radio to do helicopters, Make sure the radio has this capability some models are only for airplanes
and some only helicopters.