Haven't been on the 08 Premier which is supposed to be lighter, but I have been on both 07s and older versions of both. The 07 Premier was a lot heavier, but Hyperlite is supposed to have made the 08 lighter, not exactly sure if they've changed the shape at all, I think they just made gave it a thinner profile to cut some weight.
IMO pop and speed on both boards are very similar, Premier may be a tad faster. I went with the Premier, b/c I like the way it tracks finless. The main differences are the weight and tracking. The Substance, IMO is a little too loose to ride finless, but not overly grippy with a center fin in. The Premier, IMO is too grippy with a center fin, but perfect finless. Either way, both are great boards, but you should demo if at all possible.