The 975 and 955 are binned differently, basically meaning that they are the same chip in production but spec out slightly differently. Strangely, the 955s tend to be the fastest OC of the rest of the bunch (965 and 970), just as the 555 unlock at a better rate and OC better among the 2 core Phenoms.
I have two 975s and three 955s (one being an unlocked 555 that is actually the best OC of the three), and the 975 gets a better OC with less voltage. Both of my 975s will hit 4.4 on air stable @ 1.49v, but run comfortably at 4.3 and 4.2 @ 1.42v respectively.
The 955s top out at 4.1 and 4.0 respectively, but it takes 1.5v to get it stable at 4.1 and it runs warmer than I would like that way. Both run nicely at 3.9 @ 1.42v. Surprisingly, the unlocked 555 will run at 4.0 @ 1.42v stable and stays cooler than the other two.
But, from my experience, the 975 does OC better than the 955. The few reviews there are of the chip indicate the same.