For some reason I hate when people say this and stand by it. And I like both bands. To me they aren't all that similar and Antediluvian really doesn't deserve to be considered a "Portal worship" band or whatever it implies. It isn't Mitochondrion.
Avant-prog with a massive dark/doomy atmosphere and some surprisingly heavy bass guitar-driven sections. So much better than the Magma and other "zeuhl" I've heard.