Avantasia, without a doubt. I like most Edguy, love some of it... but it's the same formula, and pretty standard power metal. High end power metal mind you, but there's nothing that makes it stand out from the likes of Kamelot, Primal Fear, and a dozen other power metal acts I could rhyme off.
Avantasia on the other hand is where Tobi gets incredibly creative, takes risks, and is able to spread his compositional wings. Avantasia is an automatic pre-buy. I even flew to Quebec for a weekend to see them - surely wouldn't have done that for Edguy.
People used to say Edguy was gonna be the new Maiden. But with the rise of Avantasia, all the focus has switched away from Edguy. It sucks.
Edguy does more then just plain power metal these days. Hellfire Club mixes a lot of traditional metal elements into their music, comparable to Maiden and Priest, Accept too. That was the start of that element to their music. Then rocket Ride and AoJ had elements of 80s rock/metal and pop rock.
Take a look at their Space Police album. The Realms of Baba Yaga is a power metal song, but sounds like no other power metal I've heard before. I want to see more of this unique metal but with all his focus on Avantasia it doesn't happen as often anymore.
(I'm also not into the whole Opera album thing)