Why would you even bother comparing their vocals? Its like the old "apples and oranges" thing. They're completely different. Just because you seem to prefer Simen's, doesn't mean they're superior. Don't get me wrong, I love Vortex's voice too, but if you've heard him live, its not always quite as good as the albums. And sure, on more recent albums, Shagrath's vocals have been frigged around with alot(which I don't like). But with past albums being proof, along with live performances, he does just fine without the layering and studio magic.
I guess what it comes down to, is that Dimmu Borgir just wouldn't be Dimmu Borgir without Shagraths vocals. Noone would recognize them as the same band if the vocal position was changed so dramatically.