You read my mind 'cause I think the same about Mifune and Nakadai. First one is pure power and Nakadai very versatile.
For a long time I had a problem who's my number one but Nakadai won 'cause he's more calm and concentrated. Mifune was so full of energy that sometimes I thought he'll jump out the screen. Mifune was born for roles of ronins, very energetic people. Nakadai is more versatile, he can act every role, sometimes a bit too calm but really, his Tsukue in "The Sword of Doom", really, he was not the person who I'd like to meet face to face. Nakadai have also this cold attitude which is great to play evil roles (and as far as I know, in real life he's a very nice and sweet person, what you can even see from his pictures). So for me Nakadai but it's purely my personal opnion, it's very hard to pick and decide who's better 'cause it's... imposssible. They're the best on their own ways and off the screen they were friends, not rivals.