Well , I'll revise my opinion.
I would buy the Sloop O'war first and max out the cannon skill round shot, and already you will have a ship with hard attack power, then if you want even more damage max out broadsides for your sailing skill and max out barrage for cannon skill, if you want a super fast sloop for questing or just running away from explosives, max windcatcher and full sail as well.
Then with all this taken into affect, you can sink a Ogre or Monarch in 1-2 broadsides.
Even if Sloops don't hold much cargo, their ability to return to port quickly makes up for it's lack in cargo, meaning you can fill with up 14 pieces of cargo, return to port, make sail again all in the same general time it would take a Galleon O'war to make 1 run leaving you with 28 cargo instead of 18 cargo.
And as far as I know , there is only 1 ship that can top the Sloop O'war, and that is a Frigate O'war, with their array of cannons, but still the Sloop O'war is a great ship to buy and I recommend to get it as your first war ship.
