I have one of each in 12-26. There is a very small (~10 grams) weight difference and an equally small (~$10) cost difference - probably about average in the cost/weight savings equation. Also, the 970 has a shiny lock ring, the 950 a black lock ring, and the 970 has these cool translucent orange spacers between the sprockets which are eye catching. I don't know what features cause one to be slightly lighter than the other. Otherwise they look, install and function the same. It's reasonable that the cost difference is worth the difference in weight, but mostly appearance. I bought the 970 because I found an online closeout type place that had them on sale for about the same price as a 950. Otherwise I doubt I'd have bothered, because like I said, the advertised (and actual ) weight difference is virtually negligable.
I "think" my 970 shifts a little better, but that cannot possibly be because of any physical difference between the two. It's more likely either because the 970's a little newer (~500 miles newer), or because the derailleur is slightly better tuned to it (they are on different wheels/hubs and there could be a tiny, tiny difference in the way they fit in the frame).
The 950 I bought recently has the Power Glide II lock ring with PG970 printed on it. I've seen other 950 cassettes with the 970 lock ring too. It confused me because the sealed box had PG950 on it. The spacers on the 970 are orange right? The spacers on my 950 are black.
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