Katatonia are utterly different from all of those bands except Opeth, and they are still very different than them. From Saw You Drown (not that you will find this release) to present day the vocals are all clean, but they aren't high-pitched or anything like most prog or power metal bands. Katatonia are about sheer emotion. They are not technical, although they are all accomplished musicians. Jhva Elohim Meth through For Funerals to Come.... are doom-black, pretty much; Brave Murder Day and Sounds of Decay are doom-death. Dance of December Souls (first full-length album) reminds me quite a bit of Opeth's Orchid. Their sound now is not really metal anymore. It's just its own thing. Don't expect shredding guitars. Except the earlier stuff, don't expect 10+ minute songs. Don't expect complex song structures (except the earlier stuff, which tended to go through more movements). Expect very dark, affecting music. Katatonia are far beyond (METAL -- er, sorry - Strapping Young Lad outburst) nearly every band out there, in my opinion, just for the fact that they pack so much emotion into EVERYTHING they do.
But don't take my word for it. Just take the plunge and drop that 15 bucks on anything that says 'Katatonia' on it and hear for yourself. And give it some time, too -- it's very likely you won't be blown away on the first few listens. My first Katatonia album was Viva Emptiness, and while I really liked it at first, it didn't quite sit right with me somehow, and I went for a while without listening to it, until I got Tonight's Decision, which I liked a lot better. And then I got addicted -- you can go to the thread 'katatonia collection' and see my addiction for yourself (then you can look at Deliverance6's and realize that mine ain't shit in comparison =p). So, yeah. I don't recommend anything in specific. Just buy any album of theirs.