If I had to choose between those two to put on my team, which is overall a better Pierce girl? Or even just a better girl altogether.

Robinia is faster with higher attack, but has less HP than Mountain Lily with similar defense.
I'd say in terms of usefulness they're about the same, but Mountain Lily's ability is better imo because it buffs the whole party.

My pick would go towards Mountain Lily. Her attack boost is much better, 20% attack boost to everyone for the first turn while Robinia is only 15% to her and 1 random other. That's basically 100% for 1 turn vs 30%. Unless the fight takes 4 or more turn, you won't even deal as much damage additional compared to that first turn assuming the buffed target stays alive that long.

However, Robinia's overall stats does seem much better, assuming both are maxed with max affection. Though that doesn't make up for her ability compared to Mountain Lily. Unless you're doing some map with a lot of pest nodes, Mountain Lily is better IMO.

Robinia apparently gets a bloomed form, so she might be better after that. Though IMO, even with her bloomed ability, it doesn't match up with Mountain Lily's ability, but her difference in stats in bloomed form might make the difference.