Unless you're gonna convert to mass air, with the kinds of mods you're limited to with SD, the choice between the two you've listed isn't going to make a big difference. I'm with the other poster - if I were sticking with speed density, I'd probably go with a used GT40 variant (Explorer, Cobra, GT40) -- they will perform just as well if not better in the rpm range you're gonna spend most of your time in with the SD system.