save embers for Santa alter or raise robin and david now

Personally i will raise santa alter since i already have good archer, but to raise one servant just only for 1 event is not worh it. just raise any servant that you will need in long term

Taking a look at the guide on cirnopedia, the free quests have more caster and rider enemies and hardly any sabers, so raising David and Robin just for this event won't gain as much as raising Santa will. Probably stick them in the back of the roster as is for the bonus if you can farm with your front line.

This event's free quest is recommended to use AOE rider (santa alter) to farm. Also honest speaking, most event's free quests need good AOE servants to farm items.

You can raise David to unlock his 2nd skill. Santa also recommended, if you dont have Drake and Big Alexandra, or leveled Marie and Martha. Her NP5 damage plus Mana burst is quite ideal for farming. You also need AOE rider for daily quest.

If you raise her to a high level of skills, Santa Alter is either a good semi-support servants (like to Drake, Raikou, Quetz), or decent Buster DPS for rider critical team.

I haven't looked at the guide for this event yet, but if it's just for a drop bonus, then can't you just stick the level one servants in your back row and still get the drop bonuses? There's no need to actually fight work the servants, unless they also gain some type of damage bonus.