@t_wah: The higher the rarity of the weapon, the more familiarity you can gain with it. Which means straight up more raw damage in the end. So yeah, you should go for exotics (purple). They also have more skill slots. It does indeed get very expensive in terms of gold to keep it leveled, but as I said, it is by far the best weapon I have when I do.
Aah, I see. I think the fact that higher rarity = higher max familiarity and stat slots are the main thing I wasn't getting (I have often got purple drops that have only 2 familiarity slots and/or 2 skills). Will definitely give this a try the next time I boot up Nioh! =D Thanks.