O2 for your then, since you are looking on the long run.

O2 is the better GB farming/grinding ship
BSM is the best ship for bb1-3 games and still is awesome in GB but not that awesome os OPro2 

Opro Line:

Better CAs

Better AA

Better BB1/2

More scout space @ CA/BB1

More DP

Opro 2: (considering 14.96 and not 11")

+1 support slot

More firepower

+2 knots faster

More DP

Bismark: (considering 14.96 and not 11")

Better Spread

More range

I prefered The PPro2/Opro to the Scharn/Gneis but they are all good ships in the end. Bismark and O2 are pretty much even in my eyes and I enjoy them both. In a 1v1 situation between Opro2 and Bismark I would say Bismark has the advantage but Opro has its advantages too. Theres nothing that dosnt say you cant take both though.

Also fyi reguilar KM bbs are much more enjoyable then Andrea Doria.

The O-Pro line has better CAs and BB2-3, by most standards. However, the CAs, while some of the best (period) with high level crews, are a real chore at level due to sight range and spread issues. The Hipper/Eugen are much more useable at level, but quickly outclassed, and I'd say that, with firepower being an issue, the Scharnhorst's advantage there offsets the AA.

The big advantages to the O-Pro II are the extra support slot, and that it has more viable setup options, allowing some variety. The main disadvantage is slightly lower range.

Another thing to note is that the EBB3 for KM is just a slightly improved O-Pro II (extra aircraft space, extra support slot). In other words, if you can get an EBB3, you can have both by going Bismarck line, whereas going O-Pro line, you can only get more of the same.

Why 1 line, u'll need to have 2 BB BO's to lv up the proper crews for H44/Kaiser.

I played both, Bizzy run slower and smaller crew number(6 vs 7 slots) than O2, but it has smaller hit box than O2 and better at sling-shooting(turret position).

Personally, I find the O2 far more enjoyable to play. However, I think the Bissy better prepares you for KM BB playstyle. The O2 is a rushing ship, and that is definitely not ideal for most KM ships.

As Alex said, get the Bissy to get a grasp on how KM is supposed to be played at H44 Level. It's going to be a pain, you are going to hate it, you might even feel the need to slap them in the hull... But in the end you will only remember those times when you took an Alsace head on and won. :P

So, Bissy for you and then if you feel like trying something different go the Opro line for the CA's, not the BB3 since the most fun part is when you have a Deutschland with lvl 90+ gunners and support crew.