Generally radios with more channels have more mixing capabilities. I had a 6 channel radio to start with, and while I technically I use 4 'channels' to fly my planes, having independent control of separate aileron servos and separate elevator servos required me to bump up to a 7 or 8 channel radio, even though I'm only using 6 of the slots in the receiver.

The 7 channel Futaba and Spektrum, and the 8 channel Airtronics, will have similar capabilities. Spektrum/JR has the most versatile selection of receivers for different applications, though the others are sure to expand at some point (hopefully sooner than later).

Again, I really like my Airtronics RDS8000. It's really easy to use, can do a lot, and is inexpensive. The one major feature it does not have is assignable switches.