RE: P51 PTS or Arrow trainer
Either one will make a good trainer. Have you spent any time on an r/c simulator? This will greatly enhance your enjoyment and speed your training time.
Some instructors, mostly the old guard, seem to be intimidated by using a low wing plane for a trainer, but the PTS flies very easy and slow. It also has the advantage of making a very good 2nd plane and aileron/aerobatic trainer.
Good luck!