As far as sheer damage, siege frigates are the better deal than capital ships. The catch is fragility; siege frigates drop like flies if attacked by fighters or scouts, and are pretty vulnerable to just about everything. As I've said repeatedly, it's well worth your time to invest in a tough siege capital ship.
Yeah, I know. Siege frigates suck. But, often you have no choice. Still, I am looking for some direct comparison, as to which is the quickest? Maybe late game, when you've begun to overrun the oppenent, there's little opposition left, and you need to take out planets quickly.
Advent definitely got shorted when it comes to killing planets. They don't have much. It takes too long to level up a Revelation to level 6. And even then, it is still slow against weak planets, so you are forced to go with frigates. They've got nothing else. Except possibly early access to culture, which takes ~25 minutes to kill a homeworld (allegiance change capped at -.07/s). And culture seems to be easily countered.
I don't see the point, just go with the Marza.
Sometimes you can't. I think the Revelation's level 6 Provoke Hysteria needs a buff. That's part of the point.
Sometimes you don't have access to the seige abilities, like the Egg's level 6 yet. Or can't build more Cap ships. So I'd like to know what the frigate's "damage" actually is? Annatar? Or Kitkun? You know?