Speed doesn't really matter: a character with a Speed stat of 28 will get exactly the same number of turns in a battle as a character with a stat of 30. A higher stat usually lets you act sooner at the beginning of the battle, but that's it (and I wouldn't normally want my white mages to be acting first anyway). So if you are going to base your character selection on stats, it basically comes down to 1 Magic point and 1 Magic Stone vs. 2 Spirit points - nothing major.
I prefer to select based on abilities: Dagger has the better Eidolons, yes, but I personally never use them, and Eiko has a much, much better white magic list. Here are the spells Eiko can use which Dagger can't:
And here are the spells Dagger can use which Eiko can't:
Scan, Confuse, Berserk, Blind
It's just no contest: Eiko stomps all over Dagger. Eiko has some of the white mage spells like Esuna and Dispel, while three of Dagger's four unique spells don't even have an effect on most bosses. Add to that the fact that Double White is a stronger Trance ability than Eidolon and it's clear Eiko is the better character choice.