Which weapon refinery would be best for Ephraim?

Refinements add effects to a base weapon and cost 200 divine dew.

Evolutions result in different weapons and cost 150. The same happens with Durandal, Aura, etc.

I'd refine Siegmund, it allows him to still work as buff support and makes him stronger. Flaming Siegmund is strong too, but Legendary Ephraim can already use it and while it allows him to double in both phases, it still has position requirements. Refining regular Siegmund also gives him +3 HP.

Depends on what you need. +Eff allows him to stay a super buff unit and double on playerphase whereas Flame removes his buff potential and replaces it with potential double hits on both phases. I feel Ephraim is significantly better with access to his buffs but if you want him to be a position based mix phase unit you can try it.

Flame Siegmund is more worth it. Your Ephraim is tankier on the def side, so he could take better melee hits and be a better duelist. Flame Siegmund also allows Eph to run Vantage and still double both phases, while +eff Siegmund needs you to run QR in the B slot. This is assuming you'll go with the Aether and Heavy Blade seal combo.

I'd say go for +Eff. If you give him Flame Sigmund he just becomes a budget version of his legendary form, with his unique effect he becomes a great buffer who doubles as a bruiser.

As I always do with Ephraim topics I highly recommend Atk/Def Bond as his A skill if you ever summon a spare Fjorm. It solidifies his role and buffs his two highest stats, I particularly like it with Vantage as unlike Death Blow the Atk boost is both phases.