Porcupine Tree's musical catalog is actually somewhat diverse so it might depend on what "phase" of their career you think you'd like best. What song did you hear from them on XM?
Anyway, if want an introduction to early PT and a very prog-rock, psychedelic sound flavored with Pink Floyd and Tangerine Dream, I'd suggest "Up the Downstair" (their 2nd full album); if you think you'd like a more mainstream bent with hints of Radiohead, try "Stupid Dream"; and if you want to hear the best representation of what PT is today combining psychedelic-rock, atmospherics, pop and heavy metal angst all geniously crafter together, I'd suggest one of their more recent albums with "In Absentia" and "Fear of the Blank Planet" rising to the top in my opinion.
In any event, you can't go wrong with any of them and I would suspect that after hearing one or two PT albums, you'll be buying ALL of them soon anyway!
Totally agree with J Perez . PT and especially Steve Wilson , have changed the musical style over the years , even with Steve doing solo / other projects . Early albums are very keyboard and Floydish .Later , more guitar work and increasingly "heavy" , though not in a bad way . I enjoy the whole spectrum of what the band has done and continues to do . Start at the beginning and hear a musical journey that is still progressing . Really recommend "Voyage 34" , "Lightbulb Sun" , and "Fear o.a.b.Planet" Happy listenings and explorations . You'll be at a gig like the rest of us very quickly .