I played 9's for like the first 20 years I played. I switched to 10's after getting a free case of strings, and then used them for the next 20-ish years, without questioning either why I stayed with them or if they were preferable. I was probably lazy and just didn't want to set my guitars up again, haha. Well, 4 or 5 years ago I tried 9's again and it was like a revelation: they feel better, sound better, have a tighter low end, let me play with a lighter touch, are more dynamic, tear my fingers up less, and did I mention they sound better? Honestly I find myself wishing I had never stuck with the 10's all those years, because switching back to 9's has actually improved my playing.