if you've been grinding through the coin system you can buy the Hyuga (EBB3) with 80 coins.
That aside, and if you can't get it for some reason, I would say use both! The Nagato is probably the best BB3 and very good speed/range combination but the B65 is IMO a better levelling ship since it has an extra support slot and PBB bonus XP rates.
how are your engineers? the B65 can hit 46 knots capped but is hard to cap. I'm still trying to cap mine but I'm looking forward to some high-speed fun.
I would also say that with your gunners, stepping back to the Fuso or Ise is a perfectly valid option. Stick the second set 16" Ds on it and go nuts.
To be honest, if your crew is already lvl 80-89 the PBB might not be a great help unless you just want to have fun. The Fuso/Nagato with that crew will own and won't have as big an XP gap due to level difference.
If your supports are behind or if you are lvling AA gunners and want to practice AA before Yamato then yes get the PBB.
sorry if that's confusing, I had a lot to say because I was in this position myself but frankly with those gunners you are in a good position, IJN has somegreat BBs for your level so just play all of them and see what you like!