This is surprising to me that the K9 has held up better than the Sunline for you. I bought a spool of the 12 lb K9 for my cranking rod and I've really not been very happy with it at all. Doesn't seem to handle and better than any other despite what everyone says, and I've had more than one breakoff inside the reel with it. Now I'm sure that was due to a backlash or something at some point, but I've not had Sniper break off inside my reel despite having been through more backlashes than the relatively fresh spool of K9 was when it broke.
Have you noticed their higher lb tests being better than the lower ones from K9? I broke off my first ever fish on the hookset while flipping 25 lb Sniper as a leader on 50 lb braid the other day. Well, first ever one that didn't snap easily and leave the telltale pike fray at the end, at least. I'm fairly certain I had nicked it somehow though as it seemed to have broke right above where the sinker had been pegged.