The radio system can move up to the next (and later) aircraft.

The cost of a 4 Channel system is VERY close to the same as that of a 4 channel "flite pack" which would have the 4 servos RX and airborne battery. In essence when buying a 4 channel system the transmitter adds no cost over the flite pack.

If you know you plan to eventually want to fly more than one airplane the same day... you may as well start witht he less expensive 4 channel set... then when you have built up your modeling budget... get the better radio system second. The radios will get better... and you may find you want more than just a 6 channel set, so this can save you money in the long run.

The 4 channel radio can be delegated to sticking with your lesser aircraft... or you can use it for a buddy box if you get the better system on the same frequency. (so the reciever of the 4Ch system will work with the better TX without getting a new crystal.)

The basic 4 channel set will teach you to properly set up the control system rather than relying on the computer to correct for a bad setup. Learning the bad habits the computer system can let you get away with would eventually catch up to you and start costing you some aircraft.