[Thinlómien]

I'll put now my word on the topic.

Galadriel and Lúthien can't be compared. Not only because Lúthien is half-"divine" and Galadriel is "just" an elf. The other reason is that their power is quite different. Lúthien's powers are more in "personal level"; her powers have great effects in eye-against-eye -combat or in things considering herself: she makes Morgoth fall asleep, she drives Sauron away, she moves Mandos, she grows her hair by a spell, she's absolutely charming... See? All her "spells" or powers concentrate on a single enemy or on herself.
Galadriel (with Nenya) has smaller, but larger-scale effects. She ensafes Lórien and protects it by her and the ring's magic. She can see future, past and present (with her mirror). She is "an affecting world power", if I can say so.

And one more thing that came into my mind: Galadriel may seem more powerful, because she is more powerful due to her environment. In the Third Age (when she's in the height of her powers, mostly because Nenya) the most powerful being is Sauron, who's a maia. When Lúthien lives the most powerful being in M-E + Beleriand is Morgoth, who's a vala. And in Lúthiens times there are more powerful people, fg. elves are generally more powerful.