I had the same dilemma (wanted the ~6.5 sail though) and decided to choose Remedy'08.
I had 7.5 Matrix'06 before and didn't like it (too unstable), changed to GTX 7.5, loved it and got 6.0 GTX which is then rarely used (too small for my typical local conditions) and when it blows for 6.0 I use a smaller board and have difficulties to waterstart a 3-cam sail.
So I changed my strategy and went for a lightweight and easy-to-maneuver sail. Matrix has a fuller, lower foot so IMO it's better for blasting, but more problematic in maneuvers.
Matrix is more slalom-freeride, but my dealer also said that '08 is a completely new sail.
Remedy 6.4 goes with 430 mast which means I don't need new mast for a 5.6 Remedy in the future (Matrix 6.5 is 460).
Hope that helps, please keep in mind that I'm an intermediate/beginner myself.
P.S. Question on the side: GTX 6.0 vs. Remedy 6.4 in terms of early planning?