I take the sword. I never summon espers in the game, but I usually go through the colliseum and get a few merit awards, so it's useful having a strong weapon. Ultima (or really magic in general) isn't really important in this game to me. As soon as I get the cursed shield I equip it on Sabin with a ribbon and then continue on with the game until it's uncursed, but afterwards I never really shoot for teaching people magic off it. It's just a high quality shield.