If you own and like the B65, I wouldnt bother with the Mutsu. The Hyuga is just great with the 6 BB3 duals, it easily beats the nagato and with an extra support. The Mustsu is really just a nagato with an extra support, extra bind of ammo, 250 more DP and more spare displacement. It does also have access to the BB FCS 2, unlike the nagato AND hyuga, so it has the best accuracy of the 3, but compared to the Hyuga it also has 2 less guns.
I'd recommend the Hyuga over any other IJN BB for leveling, fun at 120 and because its a bit special (BB2 sprite with 6 BB3 guns), where I'd only really recommend the Mutsu to collectors or people who are really badly struggling with the Nagato and need a spread and support bonus.