No, the SB does not have more sensitivity -- at least I don't think it does. Both the S and SB allow the meter to work down to 10 seconds (the AS does also). The SB was a late improvement of the S -- more compact, for one thing. Nikon got rid of the S's large hump on top. But the SB wasn't made for very long before the AI mount came out. Enter the "A" and "AS" finders (DP-11 and DP-12, respectively). The SB and AS are virtually identical except for the lens mount. So, because of the SB's short product run, it is the most uncommon of the F2 finders, and often fetches a premium because of this.
Despite the S's rather ungainly look with that big hump, I like it. Like swchris, I prefer it because its readout is bigger and thus brighter, so it works better in bright sun conditions.