Seigbert or Ares?

Both are really good, but my vote is gonna go toward Ares. (Though, why not have both ready in case one situation favors Siegbert and the other requires Ares, since you were already thinking about promoting Ares?)

If you ever plan on going for an Ares merge project, definitely promote him. Even if you don't, he's still really good and worth promoting because Dark Mystletainn (and its innate Special Spiral) is that good of a weapon. Constant Bonfire or Draconic Aura procs are so good once you have it set up, especially if you run the Vantage set with Double Brazens or Distant Counter + Brazen seal. He's incredibly potent on both Phases, whereas Seigbert leans much more toward Player Phase play.

They're pretty similar...Ares is a bit more flexible once he gets going because you can focus him on player phase using an AoE special and it's pretty devastating or you can use vantage for dual phase (and even DC is not out of the question; a big bonfire or draconic aura will kill even blue mages if they're not tanky or HP-inflated like in PvE so he can protect himself that way), but the challenge is getting him going. I suppose he's also more consistent because he doesn't need to double so hyperspeed units can't use their spd to save themselves and wary fighters or other units who depend on forced undoubles won't get any help that way.

Generally speaking, I find that the meta is shifting a bit more towards OHKOs or faux OHKOs (ex: Surtr 20 chip damage + debuff into one heavy hit). Special spiral was provided to infantry and armours; Cherche gets an 11mt brave prf; we get units like Ophelia; null follow-up becomes a thing; Surtr becomes a thing; Hrid comes in with a conditional omnibreaker; Eir has a forced undouble but very low physical bulk; whitewings can conditionally turn their 16mt weapons into brave weapons; Lewyn's on hero fest (and as a green no less). Spd doubles are anything but dead and they're still effective (especially in PvE), but they feel a bit old-school (which is not to say they're bad), and Siegbert's natural double belongs very much to that style.

So it's a stylistic choice, and the last thing to consider is that Ares would cost you 20k feathers.

If you want offenses Siegbert, defenses Ares. I like Siegbert better but I dont think they are interchangeable

Vantage+Luna Ares has the nice ability to counter Surtr once his special is charged. Run in, smack Surtr with Luna. Enemy phase, Vantage smacks Surtr with Luna again. This kills the crab.