I have an 04' Murray and I just bought the 05' Substance. Alan is right, the boards are really nothing alike. The Murray was a great board for me as a beginner. It rode well switch and the pop was always consistant. I always got the same pop off the board each and every time. The Substance is a 3-stage rocker and the pop is much more abrupt than the Murray board.
With that said I like the LF Substance more than the murray. I ride the substance finless and it still tracks very well. It also does well on transitions when cutting, the board locks in well and cuts hard to the wake. On the surface without the fins the board is easy to manipulate and do surface tricks with. With that said the pop is more difficult to control in my opinion. This is partly due to the fact that I just started riding it and partly due to the fact that I have never ridden a three stage rocker board before.
If I were you I would go with the substance.