Neurosis and Isis are definitely two different beasts even if they are both Post-Metal giants. Personally I think Neurosis are closer to the Sludge Metal side with an emphasis on slow crushing riffs whereas Isis is more a traditional Post-Rock band infused with a heavier sound. The Eye Of Every Storm is probably the closest Neurosis have come to that "light" Post-Rock sound, and IMO it's a really good one. Personally I started with Times of Grace and have very fond memories of it. Either that one or Through Silver in Blood would be my favorite, but you could literally throw up any of their albums since Souls at Zero and it would be awesome. With Isis, they don't have as many albums, but nobody can deny the awesomeness that is Panopticon and Oceanic.