This is the way I see it: When you buy the RTF, you will only need the field accesories (Glow plug Igniter, Starter, Fuel, etc.) there is no need for any thing else to start the fun.
But let's think that you already own an engine or a radio, then the ARF is the right choice. Also think that the radio is an entry level one and you maybe will want to upgrade in the near future.
The engine that comes with the RTF is a great one and easily could be used in a second application, as same as the servos and receiver.