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.