I've worn out my SRAM PG970 and I'm looking for a replacement. I like the SRAM Cassettes as they seem to shift better with my Shimano drivetrain than the original Shimano Cassettes. Being a cheapskate, I am looking on eBay and I am seeing a lot of PG950 Cassettes. What is the difference? I tried SRAM's website and all that they say is that the PG970 is the right cassette for my riding style, even when I click on the PG950.