You have to ask yourself how far you want to go with the hobby. A six channel computer radio is a good choice for the majority of 'sport' flyers. By sport i mean those that fly for fun. Jet jocks, aerobatic champions or heli flyers may want more, but a basic six channel computerised set is a good step up from the 4 channel setup.

The majority of computer sets (if not all) will have memories for different model settings. A set which you can type in a name or identification will make life a lot easier. I don't know about the new ones, but the earlier Futaba Field Force 6 sets only allocated a number to each memory and I was forever forgetting what number was which plane!

Another advantage of computer radios is that they will make set up easier, allowing the servo direction and throw to be adjusted electronically. Although a good initial mechanical setting is also essential. Mixing between functions is also fundamental to their operation, such as automatically putting rudder inputs in when you move the ailerons.

They also allow more complex model types to be flown, Such as V-tail, Deltas, Flying wings etc.

This is obviously only an example of their ability. I'm sure you know most of it already. Modern sets are capable of a huge range of functions.

PPM is Pulse Position Modulation and PCM is Pulse Code Modulation. PPM is cheaper. PCM incorporates a fail safe function whereby if the signal to the model is lost (interference) then a pre-programmed event will occur. Usually for a sport model it is set to hold all the servos last position except the throttle which is automatically brought to idle. You can set each servo to either hold a position or revert to a set point. (As an example model jets are often programmed to lower the undercarriage, deploy the airbrake, bring the throttle to idle and hold all other servo positions.) This function may allow you to recover your model but at the very least should stop it flying away. I think PCM is also slightly more interference resistant, although modern PPM dual conversion receivers are pretty good!.

Hope this helps. Feel free to ask more if you need more help.