Actually, I've been doing some testing on this. The Warp11 9500np bios doesn't seem to overclock as well on my voltmodded Infineon chips (I max out at 311mhz on the memory stabily) but overclocks a little better on the core (398mhz) Overall, that bios scores better in both 3dmarks.
Versus the 9700np Club bios, which overclocks a lot better on the ram (324 stable) but not as well on the core (385 stable) and nets slightly lower 3dmark scores.
To tell you the truth, I can't tell much difference in real games either. UT2K3 seems to favor the 9700np bios for most maps, but depending on your graphics settings it sometimes likes the Warp11 bios (and the lower memory speeds) better.
So whatever you do, test it extensively before making your decision. Sometimes a higher available overclock doesn't always net you a faster system...