I've done group study throughout, with private study to supplement. I find that I need different bodies to abuse, I mean, practice with. Being forced to adapt the moves to a 5'10" 180lb guy to a 6'4" 240lb guy will show up the areas to imrove pretty fast! Also helps me decide what I would use on the street and what I wouldn't, depending on who I am dealing with.
I took privates weekly for two years until I reached black belt, but now funds restrict my classes. If there is something I need more work on, I will use the private.
Also I highly recommend going outside the home base for seminars- keeps ya on your toes, your mind open etc.