The biggest factor is you: your self-discipline, how much work you put in, your willingness to keep studying even when your friends are inviting you to go party all summer. The biggest factor is NOT which course you take. I have taught for 2 different national companies, and I've seen people do very well with every prep method under the sun.
No one can guarantee high scores; that is pure advertising hype. Kaplan and TPR students can get great scores, but they also are many of the students with the worst scores. Why? Because a lot of people take those courses, so by default many of the best and worst-scoring students have taken Kaplan or TPR.
All the company will do is "guarantee" that you can come back and take the same course at no extra charge. That's not very helpful if the course sucked the first time. (The full money-back guarantee is very hard to get because it requires doing ALL the homework, which even if you had time might not be the best use of your time.)
If a course provides the structure and deadlines that you need, and you have the money, then take it. If you have great discipline and no money, you don't need a course. Buy some good books (Examkrackers is my personal favorite) and get to work.
If you had an exceptional instructor, he/she would be totally worth the $$ no matter whether they taught every section or only one. A great instructor can give you great mnemonics and explanations that the book doesn't have, give you individual advice, identify your weaknesses, and help you target your studying appropriately. There are not many of these around; you find out about them by word of mouth. A mediocre instructor will do nothing for you and isn't worth the $.