I would wait. There is not one supplement that is superior to another, it is really about personal preference. Once you get into class, see what the professor is doing and figure out your learning style, then go back and look at several of them again. You will probably be able to make a better choice then.
I would definitely lock in your rate with BarBri now if you are pretty sure you will be using them for the bar exam. It is nice to know that your rate won't go up and the free materials are nice.
I don't remember if PMBR gave out free stuff or if they even have a lock-in rate procedure. If something is free, go for it.
If you do decide to buy something over the summer, just make sure you are doing it for the right reason. This is just a thought, but I would advise against buying it with the intent of teaching yourself the law prior to going to class. If, however, you are doing it to get a good overview of the topic, I think that is an excellent idea. It is nice to see an overview before you dive in and for some people, it makes the whole experience less stressful once you begin.
Edit: I just wanted to add that if there is one thing that I would recommend doing prior to law school is to find a book that teaches you how to brief cases and outline courses. You may decide later not to do the outlining (it works for some but not others) but having those skills from the beginning will really help you.
And, aside from everything else, enjoy yourself this summer. Trust me, you will be glad that you did.