Can a Priscilla serve just as good of a function as Elise can?

Function for what? Offensive staff magic damage on a horse? Priscilla is slower and weaker, but a boon to Atk or Magic matches Neutral Elise. Cavalry buffs will help and just building a Priscilla is far sooner/more likely as an Elise Banner is not something that happens often nor is it a guarantee soon.

Objectively, Elise has better stats for an offensive build, but Priscilla is only slightly worse and has much better bulk to help survivability.

I'd say she works just fine.