Arguably keyboard+mouse is "better" because accessing things like chat shortcuts and your subpalette are much easier to do on the keyboard, as well as certain movement techs/aiming with OTS view. That said, I find keyboard controls to be pretty awkward and PSO2 is very clearly designed with controllers in mind, so playing with a controller feels very natural and comfortable. I've tried both and even as a Ranger main I can't stand keyboard+mouse for extended periods so its controllers for me!
If you're playing controller on PC tho, don't forget that you can still use your keyboard. It can be a bit awkward depending on your setup, but I find it's more practical to use keyboard for the majority of my subpalette (selecting a skill/item takes too long on controller so I just keep it over my main skills on one side and reach over to use items) and chat macros when needed. I'll also quickly swap to mouse for turret aiming, so I guess I kinda play hybrid keyboard+mouse+controller...? but uh, I might be a bit of an outlier, who knows lol.
Anyway, try out both and see what you like the most! If you have 12 thumb buttons then mouse might be easier for you anyway if you can sort out a good layout for yourself.