The skill level for both to "git gud" I believe is the same.
They are both very complex characters to be good at, I would have said SAM if you hadn't played NIN before. Because it is more forgiving, if you mess up a combo or rotation.
With the NIN, I find it personally very hurtful if I mess up something due to lag or mechanics or something.

But since you have NIN to lv 50, then you are probably used to it now. So it won't really affect you.

You like long katana or a dual dagger sneakiness style?

I like SAM and AOE and KATANA.