I think Akerfeldt's clean vocals aren't quite as good as Stanne's. While there really isn't anything wrong with them - you gotta admit they're a bit bland. They sound too much like an average light rock singer (please don't flame me for this). Stanne's voice is lower, easier on the ears. And he's got a very unique style, very emotional.
On the other hand, when it comes to growling, Akerdfeld's superior. I thought Stanne's growls were too sharp in Projector, and too high in earleir albums, plus he's got that annoying habit of inserting additional syllables at the end of words from time to time, Hetfield-style... Akerfeldt's growls were lower and generally sounded better. Until Haven, that is, which is the point where Stanne's growling voice impreved a lot, But he still puts syllables where they don't belnog sometimes....
But both of them are surpassed by Dan Swano. He's my favourite growler, and among my favourite clean vocallists.
