Cresselia is way better. The extra HP makes up for the lower SpD/Def Stat. Not only that but the CM set is a bitch to take down after its counters are dead. Cresselia dosent just sit there it functions as a very good roll at team support and if taking hits like a champ is called "just sitting there" then I'm a hypocritical asshole. Having a reliable recovery move isnt hurting Cresselia either >_>
I disagree entirely. You cannot say something is way better just because it does different things better.
I won't even get pedantic with you; I'll stick to viable sets / strategies.
Uxie's advantage over Cresselia lies in the fact that it is an effective Pseudo-Hazer (Yawn), and it has access to U-turn (making it an effective team "scouter") and Thunderbolt (making it a much more reliable counter to Gyarados than Charge Beam / HP Electric Cresselia).
Uxie also supports the team by being able to set up Stealth Rock, and it also has Calm Mind (though I will admit that it does not use it as effectively).
One significant advantage with the Calm Mind set, however, is Uxie's ten higher base Speed. This allows Uxie to outspeed common metagame threats like Lucario and Heracross without a +Speed nature, allowing for more EV / Nature investment otherwise.
In terms of support, Uxie gets Thunder Wave and both Screens, just like Cresselia. However, unlike Cresselia, Uxie gets the aforementioned U-turn, Stealth Rock and Yawn, all of which can be feasibly used on the same set, meaning I would argue that Uxie is actually a more effective team supporter.
What Cresselia has the advantage in is Moonlight (except in Sand / Hail, and no, Moonlight is NOT better than Rest in those weathers), Ice Beam and generally better bulk.
I don't see why Cresselia is "way better" based simply on that criteria (o, and the fact that it can run a funny Psycho Shift set but comon lol).
I would just go with both have very specific purposes, and both accomplish them very well.
However if you require one or the other, I would take Uxie since it is better in general in the supporting the team, and its ability to more effectively counter Gyarados is more appealing to me than effectively countering Garchomp (Cresselia) because from what I can tell, we're probably going to send Garchomp packing.