i had the first gen CS-9000 with 4 rivets on the second carrier. when my cog snapped in pieces, three rivets were totally off. the flexing probably snapped the cog. i think it was the 19T. the second get cassette has 8 rivets and seem to hold up a bit better. i lube the rivets to keep them quiet. make sure you get the newer 8-rivet carrier where for the second cluster...you should be good. if the price is the same with the 9100...i would probably go for that.
The 9000 is cheaper than the 9100. On one site I shop, $200 for the 9000 11-28, $240 for the 9100 11-28.
I have an Ultegra 6800 11-28 cassette too. What puzzled me though is something I ran into recently. I have DA cassettes on my 2 Parlee's (one is otherwise DA, the other Ultegra). I bought a direct-drive trainer, the Tacx Neo, that comes without a cassette. I put the unused Ultegra on the trainer and could not get it to be silent. I got plenty of chain noise in the middle cogs. I re-mounted, re-torqued, micro-adjusted the Di2 derailleur, futzed for over an hour, maybe 2, getting frustrated, but could not get the noise to go again. I pulled a DA cassette off a spare wheel, and it was silent. The DA and Ultegra cassettes should be totally interchangable, totally the same spacing, but I couldn't get the Ultegra one to be silent. Perhaps someone more skilled than I at rear derailleur adjustment could get it to work quietly. I could not.
I am going to put the Ultegra on the spare wheel and see if it's quiet on the road.
In any case, I know lots of people recommend Ultegra cassettes over DA due to carrier and/or creaking issues with the DA. I've not had any issues with my DA cassettes. For the 9100 it appears most of the parts are the same as 9000, including the carrier. For the few that are different, like the lockring, I would imagine that's cosmetic. The 11t is different, not sure how.