ORIGINAL: piper_chuck
If you're sure you will have other planes, a 6, or more, channel computer radio is a good choice. If you're not so sure, buying a 4 channel doesn't usually end up costing much more in the long run. The reason for this is the 4 channel radios usually cost little more than a flight pack. If you buy 4 channel now and then upgrade to 6 or more channels, the flight pack you got with the 4 channel should work with the new radio.
Chuck is right, though. A 4 channel isn't going to cost you much more than a flight pack. In fact, I bought a 4 channel after I already had a 6 channel computer radio, because I literally got the transmitter for less than $10!