Stahl!Owain is a slightly less impressive version of Frederick!Owain. Granted, he gets more speed, but stat mods are way overrated. He'll be okay at everything, great at nothing, and worse than if Fred were his dad. C
Vaike is interesting. There are two things Owain wants from a physical dad: Sol and Armsthrift. Vaike offers half of that, and probably the more important half. This is a B.
Vaike!Owain = B. Can make a pretty good hero with Axe and Swordfaire, or Dreadfighter with Axe/Sword/Tomeflair along with Sol and Vantage in either case, but I don't see him doing anything else well. That said, he doesn't need to be good at much else.
Stahl!Owain = B. No negative or neutral stats. Can be a good offense rally bot (particularly if he inherited Speed Rally or Movement Rally) and can throw Dual Guard+ in the mix for when he's not rallying. Lead wise he's bringing Luna and Aegis to the mix, which is not bad. Archer line just does nothing for him though. Vaike at least gives Owain Lucky Seven along with Sol and Axebreaker as options for leading.
And so begins a tale of vengeance and betrayal. Destiny by sinner sought. Tragedy by power wrought. Valkyrie Profile: Covenant of the Plume.