I plugged in stats of Dark Fina w/ BIS gear and Gilgamesh w/ BIS gear into a spreadsheet, and their damage is surprisingly close, with Fina doing about 6% more damage per turn with dualcast ultima. Noctis actually deals more damage with Warp Break (single target), but I'm comparing AoE to AoE.
With that said, Ultima can't take advantage of elemental weakness the way a weapon user can, can't use killer effects, and chaining ultima is next to impossible. Ultima also costs 120 mp per round, compared to Divider's 45mp per round.
Just to clear it up, don't think because Fina has a lot of elemental spells, she can hit a weakness too. Even on a target with -100% fire weakness for example, Ultima still does more damage than Firaga.
Summary: Wait a week or two and blow all those summons for Orlandu, who is superior. Both Fina and Orlandu banners are terrible aside from the 5* prize, so keep that in mind.