ORIGINAL: Campy
I suggest buying an ARF for your trainer. This is because when you build (and are inexperienced at it ) you may accidentally build in a warp. This WILL AFFECT the flying of the plane.
While all that's true, would your rather have a trainer with a warped wing or a Sig Kougar with a warped wing? Better to learn to build on airplanes that are more forgiving than put it off and then need to have very good skills at the beginning when moving to a more advanced model.
A trainer kit will let you get away with a lot of mistakes in the build that a more advanced plane won't. If you don't build your trainer, then where are you going to pick up the skills you need when it actually matters that you have them?