The 9s are a HUGE improvement.  Flexibility, and the fact that the toe piece fits into the groove should help durability.  The 8s had a problem with the sole being so rigid that the sole split.  I must have seen at least 10 pair where that had happened.

You will probably end up buying more heel/sole options, but I'd buy the 9s without hesitation, whereas there is nothing to make me buy the 8s.

The Warranty is great too.

I currently have the 9s, and I had the SST8s for 5 years prior. My SST8s finally died from the dreaded sole split. But that was after a life of a couple of thousand games, and the shoes were ready to be retired anyway.
 
The 9s are an improvement in many respects. The sole/toe replacement system is a forward advancement in shoe technology. But the 9s and the SST8s are different animals. They feel different and not everybody is automatically going to prefer the 9s.
 
Anybody considering one or the other should really try them on before making a decision. They both have their good and not so good.

If you use multiple slide soles, be sure to compare the cost of the replacements of the 9's vs the SST 8's

 
That's one of the 'not so good' for the 9s. 

I had 3 pairs of 8's..... sold one while back before the prices jumped. Anyway, I still have two of the sst 8 when the prices (discount I think) were <$120. And after all these years, they both are still going strong. I have wide feet and get a size bigger, per Podiatrist orders. I wonder if also getting the wrong size shoe promotes early wear and tear....