Psyshock is better in general though, Thunderbolt only lets you hit Lugia harder while Psyshock hits everything else TBolt does and more.
Another gen, another gen for Lugia to get worse in.
Also, for reference: (yes I counted Fairy Aura)
+2 252+ SpA (custom) Psyshock vs. 4 HP / 252 Def Eviolite Chansey: 361-425 (56.23 - 66.19%) -- guaranteed 2HKO
Ice Beam: 32.24 - 38% Focus Blast: 54.2 - 63.86% Psyshock: 56.23 - 66.19%