I think their sound is closer to Tool but they might appeal more to Porcupine Tree fans. The reason for this is the style, the overall feel of the music. Tool is far more aggressive, I'd say. Dead Soul Tribe is heavy but relies more on atmosphere than intensity (compared to Tool). In this way, they are similar to Porcupine Tree.
I think one of my favourite things about Dead Soul Tribe is how effortlessly their music flows from idea to idea, nothing ever feels tacked on. With a lot of progressive metal bands, I find they try to do this or that to give a song a unique feel (in the intro) and then it just leads into a very standard, uninteresting track. Dead Soul Tribe doesn't have that familiar sound, they manage to have a feeling that I could not ascribe to any other band. And the lyrics are great. Bad lyrics don't turn me off from a song, but good lyrics can enhance a song.