But if Lucy can use her star dresses to cast Urano Metria, and her strongest spirit is Aquarius.....Then doesn't that just mean that she'll always be stronger in Aquarius star dress in terms of attack power?I think you are missing the point. Stop using Urano Metria to asses Aquarius stardress because because those are two seperate things. The only spells Lucy has in Aquarius stardress to date is Water shield and Aqua Metria.
By your comparison it would be a nuke(Starshot) vs Avalanche. A bunch of nukes are far more damaging than an avalanche, Both would cause a lot of damage but you have a higher chance of dying(Clearly not in fairy tail) than by being hit with a mud slide. It is similar to how Fairy Tail mages can tank waves of water but they can't take lasers or swords piecing them. Also a regular arrow from Sagittarius can pierce throw Metal while a huge wave from Aquarius couldn't piece through rock.
Aqua Metria is powerful but less likely to actually damage a mage where as something that has piercing power combined with explosive power is more likely to damage an opponent. It is like how Kagura can tank a huge explosion but is darn near fatally wounded by a stab.
What I'm trying to ask is whether you think star shot hits with more force than Aqua Metria. I guess a more relevant question would be who do you think starshot beat that Aqua Metria can't?