As Radiohead has moved progressively farther from pure rock (and yet continued to retain their musical perfection through almost every innovation), Portishead has moved closer to pure electronic music. When you look back through their respective catalogs, this evolution is not as surprising as it may appear at first.
On Pablo Honey, Radiohead made English grunge (that was more refined than it's American counterpart), but played with different ideas and sound patterns. "Thinking About You", "You", and "Prove Yourself" all contain elements of various genres - musical and lyrical explorations, if you will - that will be furthered on later Radiohead albums.
Dummy, Portishead's debut, is the genius integration of Massive Attack, Moby, and Mazzy Star. In other words, we have creepy, cool, and incredibly beautiful sounds being created by Portishead in the early nineties. Yet, although the album is full of electronic music (with the requisite repeated beats and samples, and computer-generated sounds) there's much more to this album. Five of the eleven songs have strong Jazz undertones, in both the lyrical design and the actual musical sound. Tin Pan Alley and the dirty streets of "unfriendly cities can be heard in the most popular songs on the album: "Sour Times" and "Glory Box". This is not 1980s electronic music, with clean synthetic noises and fake-sounding drum machines, this album is layered and a little messy. Still, all of the elements of electronic music are available, they just aren't being consistently utilized by the band.
The potential was always there, and the bands took advantage of their potential, moving ever closer to each other and farther from their starting points. Listen to In Rainbows and you'll hear a lot of piano, and some definite electronic influence. Radiohead's sound is still "refined" and far from messy, but they are now even closer to Portishead and quite far from Pearl Jam. "15 Step", "Numb", and "Videotape" each repeat one or multiple "hooks", taking advantage of a longstanding trait of electronic music. Piano sounds also come out in full force on In Rainbows.
Third by Portishead (a band that has been quite less prolific than Radiohead) was released today, and has a slower tempo and a darker tone than Portishead's previous albums. "Nylon Smile" wanders and has little lyrical structure. It is pure lament, albeit with a crackling backbeat. "The Rip" is almost freak-folk, and quite as haunting as the In Rainbows equivalent, "Videotape". "We Carry On" and "Deep Water" are musical opposites. One is long, the other short. One is fast and anxious, the other is slow and meandering. "We Carry On" has a beat that is constantly defined, and is the backbone of the song. "Deep Water" is the more melodic home to a humming chorus. "Machine Gun" sounds incredibly computerized, and quite precise. It is probably the song that breaks most from Portishead tradition, and it makes me wonder if the reason Radiohead and Portishead are sounding more alike has more to do with the emotional tangibility of the times (these are scary, hectic, anxious times), than the "natural" evolution of the artists.
Whatever the reason, Portishead and Radiohead have created albums that are complementary. They both tap into anxiety, fear, violence, and creepiness; they both question the value and purpose of the individual; and they both tap into a repetitiveness of structure that is the foundation of most electronic music. They're also both worth a listen, if you have the time and the inclination.