I listen to The Chaos Path more than any of the other songs on LMI, but for the songs that Garm sings, I don't think Simen does them any better, but maybe that's because he performed them live (I have the DVD). Basically, Garm is perfect for the songs he does. I also like Simen's performance on The Chaos Path better than pretty much any of his performances on Sideshow Symphanies.
Zeph you might know the answer to this, who writes the vocal lines? I assume on LMI, Garm did it all, even on The Chaos Path; and that Simen did it on Sideshow Symphanies. Maybe that's why I like Simen better on LMI.