With her NP and skill upgrade, Jalter is a better option imo.

I would say whichever one you like more, but if it's for gameplay, Draco is significantly better as both a DPS and farmer at I assume NP5 2K fous if you're going to 120. The Grails past 100 make little difference though, so I'd definitely go for your favorite.

If I say that my attachment fluctuates between them from time to time.

According to my experience, if you don't know who to choose, don't choose at all. Save your grails to your "must Grail no matter what" servants. But you do you. You know what to do.

Jalter is always better.