I believe there is no one particular answer to this question; rather it depends upon the modeler.

If you're mostly interested in learning to fly, then a RTF airplane package will get you flying the soonest.

If you really like the idea of building your own models, modifying them and experimenting with them, and also flying them, then building a kit right out of the gate is the best way to go.

If you are interested in both and/or you're very particular about what you get for your first plane, buying an ARF with a seperate radio system and engine will give you a good deal of flexibility, expose you to the basics of airplane building, and still allow you to get flying fairly quickly.

Again, which route is best simply depends on your primary interest in the hobby.

Good luck and good shopping!