Nursery vs helena

It really depends on what else you have. Nursery Rhyme is a selfish arts based aoe caster while Helena is a team player. What other servants do you have leveled?

They are different.
Nursery is one best offensive casters because of her 1st skill. Because of that, I think she is "not bad" than even Xuanzhang and Illya as SSR. However, if you team's supports don't have 2030 or other star generation CE, or stable source of star generation, her performance was somehow reduced.
In fighting with assassins, or assassins boss with multi-class mini-bosses, Nursery worked excellent in team with Hans, Tamamo, Waver, or even Helena herself. In a turtle team that paired with Jeanne, Mashu, Medea Lily, she is still the major source to deal decent damage.

Helena is a decent support, with good versatility and utility to support any hitter, especially for critical damagers. Helena did well if you want to charge NP for the major DPS like Raikou and Ozy, and burst up their critical offense. Her unique 3rd skill is also idea for a brave chain of Major DPS, like Gil, Shishou and Drake's critical brave chain to charge NP more faster as well as deal heavy damage. Like NP+B+A+E chain for hitters.
It is not wise to pick her as a long-tough challenge quest because her skill cool-down is a severe problem, and provides noting related to defensive buff to team like evade, healing, defense-up or debuff removal, etc; or any significant debuff to enemy like charge reduction beside of her NP (but once again, hard to charge her NP if not paired with dual Waver or in a good team to Tamamo)

Overall, Helena's job as support > ~ >>> Nursery, and Nursey's job in damaging boss >= ~ >>> Helena. Their normal AOE farming is almost the same (maybe Helena is a bit better), but Helena's NP charging is inferior to Nursey, more or less reply on the teammate.

Still, depending on team comp.
Like if you have 2030s, Nursey with Tamamo+Andersen/Waver may even worked better than Xuanzang and Illya. If you have good art-up CEs, Helena can also be a good hitter in assassin fighting.

Nursey + Helena is also a good idea, in fact.

Staunch supporter of Helena here.

+++:
Helps generate Arts chains and has a party-wide NP gauge boost for those hard-to-charge servants like Altera or Lancelot
Mini-Projection grants a considerable buff for 3 turns to all card types in the party

++:
NP works well in NP chains to help Overcharge others while boosting their overall damage with her Defense debuff

+:
AoE damage is on the high side, but not stellar
Her Quick card hits a ton of times and she helps generate stars with her second skill

-:
She's a Caster - her damage is good within her class, but meh compared to everything else
Her skills are great for farming and clearing story/free quests, but not great for drawn out fights like raids or boss battles

She'll do great work alongside your Altera, Tamamo, or Lancelot, or even Emiya/Kuro. In general, she's more useful than Hans for farming. Worse at providing damage numbers with her own cards than Alice is. As noted elsewhere, can help Alice be a real girl in terms of usefulness.

I can't speak highly enough of my Nursery. If you put in the effort to level her, she'll hit like a truck with her skills (put her with a Hans or a Waver and she'll do 80, 90k buster crits easily, while an arts chain with crits will rack up serious damage as well) while also being fairly tanky- she has a heal, solid debuff resist and a solid defense up.

Her downside though is she does almost nothing for her team- her NP lowers enemy defense and might lower charge, but that's it as far as supporting others goes. Still, pair her with a Waver and a Hans (both easy to get through FP gacha or Support list) and she'll tear through assassin bosses with better sustained damage than Illya or Sanzang (at least in my experience)

But since you have both, pair her with Helena who can provide the crit stars she needs and an arts card boost for three turns, and laugh in the face of your enemies.

I have both and I prefer Helena. I use them mostly for farming so the main difference for me is that Helena can charge the NP of the entire team, whereas Nursery just charges her own (however slightly faster).
Using Helena I can manage to pull off 3-turn farming without a Kaladeiscope.