From ZZP's site:
90# springs are GM LS6 yellow springs*. The LS6 comes from the factory with .550" of lift and spins to 6500 RPM to give an idea of what the springs were made for. Seat pressure is lower than the Comp Cams springs but spring rate is higher so they do work well for aggressive ramp rates. There springs work well with the VS, NIC, or XP cam. Because of the higher spring rate, timing chain dampener life is lower than with the 105's.
That's all i can really think of, that's why i'm probably going with the 105's over the LS6 springs.