who to ascend

I suppose it depends on what you need most.

Euryale, asides from being a powerful anti-male unit, works well for stalling the enemy's NP with her charm (both from her skill and her NP) and NP drain (which also charges her NP gauge even if it fails to drain), and also nuking the enemy repeatedly with her NP due to her high NP gain.

David is more hybrid support and DPS but he does it very well, especially his full party Evade.

Another ST archer to consider is Robin Hood. In many ways, he is a stronger nuke than Euryale. Obviously, he lacks her NP Drain and charm skills but makes up for it with a powerful NP (much, much stronger if his first skill poisons the enemy, which also lowers ATK).

Personally, and if gameplay is a primary consideration, I would be much more like to grail Robin. His nuking capability is huge. With some good support, he can do 200k+ damage, and thus would be used as a main damage dealer. While David is good too, he, as said above, is more used as hybrid support/damage. Thus his attack is secondary, and sometimes is just swapped in for his heal/evade skill, which is not enhanced at all by a grail.