I cannot say I know too much about Porcupine Tree, since Lightbulb Sun is the only disc I own from them. I picked it up cause Mikael was talking about Porcupine Tree in this interview, and he was praising them really hard, so the interviewer had to ask which of their albums are the best. Mikael answered that all of them are worth getting, but probably Lightbulb Sun would be a good start.
I got it and played it, not really expecting anything to blow me away, since maybe modern prog rock wasn't my biggest thing, but god, it was amazing! The songwriting is what matters, no matter what genre you're into. I will get In Absentia next, I guess. Looks like everyone likes it so much.
I wonder how In Absentia compares to Lightbulb Sun in terms of heaviness, production and songwriting. There is just something about LS that makes it really beautiful in a dark way.
NP: Opeth - My Arms Your Hearse