I like Daud a lot more than Corvo.
Admittedly, Corvo starts with a disadvantage as a silent protagonist but even throughout the story, Daud is this complex person who has had his powers for a long time. He's an assassin for hire, but that's what he's good at (and I find that to be a pretty cool job). What career is he supposed to take up with these abilities, the Isles' greatest courier? If Corvo wasn't the royal protector of the Empress then he would most likely be just like Daud. There's also the fact that Daud is the most interesting and complex character in the game, in my opinion.
Spoilers about the end of the game:
Spoiler
He has taken so many lives and feels regret for it. He knows that Corvo is (likely) going to go after him for revenge and accepts that. All he does is ask you for mercy, to let him live. He didn't fight or beg, he just asked. I never brought myself to kill him, not even on the playthrough were I killed everybody I could.
Corvo just felt too bland to me, his goals were to find the Empress' daughter and put her on throne. Nothing else, sure it's an important goal for the entire Empire but it's nothing complex and we don't know how he feels about the whole affair because he's a blank slate.
My opinion (as all are) is biased because more and more I'm starting to like the idea of voiced protagonists in FPSs to help you feel like you're in the middle of events without making you either a mute. Far Cry 3 and Bioshock Infinite are the two recent games that changed my mindset.