Well, do you like playing CV or BB? What would you like to play?

But, keep in mind that, unless you buy some crew, you will need to level up a BB crew. If you aren't buying a crew then you might as well level up a new BO and save your money, as well as also being able to play your CV too.

with the cv line you got a lot of ships to buy and not to be bored with every couple of levels. However the ijn pilots- especially the fighters suck big time. The dbs and tbs are ok i guess.
With the bb line you get some of the most powerfull ships in the game. However as the previous guy said if you havent got a crew good luck buying one. Keep in mind that aa gunners should be classed late. However, the grid to bb4-5 is long and painfull as IJN BBs are made of paper and you will get frustrated several times along the way.