I don't think you'd be a quitter to switch routines before the 12 weeks are up. If you are bored with your workout, chances are you will either not give it 100 % or you will give it up entirely. If you can create a workout regimen that better suits your needs and stick to it, then by all means make the change. I agree with you, I prefer to workout individual muscle groups in a session rather than an entire full body workout.

Good luck!