Cresselia's more interesting for how it manipulates damage, in my opinion. Pull damage off something that'll die anyways onto something that's then in Bring Down range? Stop your own pokemon from being KOed by translating the damage onto the opponent?
Power's a lot more interesting then it's attack, that's for sure. You'd already want to play Premier Ball with Garde LV.X, so Cressy's a really good tech. Palkia LV.X makes for a fun tech as well, assuming you have Garde LV.X on the table as well as Moonlight Stadium/Phoebe's Stadium. Actually not as hard as it might sound, again due to Premier Ball, and also since Palkia's first attack can search your deck for a stadium and discard any in play (like Crystal Beach)
Picking both of those techs though will shift too much focus away from Garde/Gallade though, so which ever seems more interesting... It's more, who do you want to beat the opponent up with? Garde's Bring Down (Cresselia) and Gallade's Sonic Blade? Or Gallade's Psychic Cut (in which case, Palkia to pull anything up is indispensible)?