Go with whatever upgrades suit your style. For example, if you want a tank, upgrade a war frigate to Copperhead VI with Offense II rigging. If you want lots of cargo space for plunder runs, upgrade a war galleon to Fortune Hunter VI with whatever rigging you want. I go on a lot of solo runs with my war sloop, so I prefer speed to get around quickly and avoid enemy fire. My upgrades are Firestorm IV and Speed II rigging.
It's all preference, really.
