RTR for starters but after that building. Don't over complicate end users. Anything other then RTR for first timers will complicate matters and make them sell it before they run it. There's my take.

EDIT: As for me, i always go bare bones if ican. Saves cost on electronics i throw in the drawer and forget about. I slap in castle as soon as i can. I can fine tune my car and having a lot of the same electronics mean more compatibility and less items to worry about. I know how each car will handle generally.