Its always going to come down to what you want, which is based on preferences.
If it was hard to save 20k (war sloop), its going to be hard to save 40k (war galleon), and even harder to save 60k (war frigate).
If you want a War Galleon, or War Frigate you are a good part of the way towards that goal already, and saving now will get you those ships faster. If you prefer sloops, then i'd say go for it, you can always save up for the galleons/frigates later.
After my latest SvS bout, the sloops seemed to fare very well, with thier speed and maneuverability. In regular game play, galleons hold the most plunder.. which is (in theory) why us pirates are out on the high seas for to begin with.
