I started with the medium, then switched to the large to get a better finger position on the trigger which helped me shoot better. But I have gone back to the medium as I think it just feels more comfy in my hand. This is for both my 9FS & 9C.
I used the medium back strap until my wife claimed possession of my 9c. So it now wears the small one but I still find it comfortable and secure in my hand. Of course I now have to sleep with one eye open.
Started with large, because it felt better...then medium...now small. For me, it is all about making it easier to conceal. I'm sticking with small for now, but might go back to medium some day. Confused? Yep.
Medium on FS, but CT on 9c. The laser grip is a little bulkier than the large, so I had to move the mag release to the right side for one handed eject, since my thumb wouldn't reach.
I have gone with the small grip on my M&P 40C since it was new. I have never tried any of the others. I also have set the mag release to "left handed".
It's nice when a gun fits you and not the other way around.