What are you looking for performance wise, more low end power or high end? Which engine are you running? There are many different RB pipes, which one do you have?
The JP-2 is a top end pipe, while the JP-3 is supposed to have the top end of the JP-2 and the bottom end of the JP-1..
JP-2 or the JP-3 JP-3 seems to be the pipe more people are going with now. I am running it and it seems to be good all around with some nice mid-range. JP-2 is a top end pipe.