I loved both, but Dawn of Sorrow is the best of the two, definitely.
Much more ispiration in level design and music, a very balanced gameplay and weapon system (love soul fusion) and a truly circular castle, where it seems everything connects to everything else. Boss battles that made my jaw hit the floor in both scale and originality, and more. Dawn of Sorrow is my favorite Castleroid, hands down.
Portrait of Ruin, I dunno, was missing something. It had lots of great points, but the extra "umph" seemed like it was missing. It seemed too much of been there, done that. Let's do the same old thing, abandon all advancement from previous games, and get a product out.
It seems whenever IGA takes the time to make his own games and sequels, a lot more heart and effort is put into it. Look at LOI, HOD, AOS, and DOS. A lot more originality and new concepts.
But revisiting his favorite previous games (CV3, Bloodlines) and it seems like the game's slapped together somehow. Soul fusion/system in DOS/AOS, the spell/subweapon systems in LOI/HOD, the axing of leveling up in LOI in favor of new techniques. All of those are abandoned for generic CV games. IGA has to keep giving us new ideas, or step aside and let someone else do it.
Again, sorry for the long post. Guess I'm in the writing mood tonight.
In the end, IGA's new characters and concepts will always win. So as I said, the answer's Dawn of Sorrow.