I don't think there's really much difference in overclocking potential. At the extremely high end (240-250FSB) you might have a few less issues with the NF7, just because it has less features to screw up. Otherwise...they're pretty much the same board.
Both the NF7 and NF7-S are basically the same board. If you have no need for SATA RAID, Firewire support, or Soundstorm audio, then save some cash and get the NF7. Both will overclock pretty similarly.