I already have M&B, but should I go for the other M&B titles or King Arthur II?
Warband is worth getting because of it's MP and SP additions, Plus mods. But Fire & Sword is passable, Unless you want to have the complete collection or it's on sale for very cheap (There are mods for Warband to add firearms)
I haven't played King Arthur though so I can't speak for it, Although I hear it's like Total War but with magic.