I saw U2 in Brisbane on Tuesday and then Pearl Jam on Saturday (second show). I'd seen U2 before but I was a Pearl Jam virgin. I've been a fan since their first album, but somehow, I'd not before made it to a show.
I've thought for years that Pearl Jam are the only band who might be able to put on a better show than U2. So to see them both in the same week was quite a chance for comparison.
There are three main differences between U2 and Pearl Jam as live bands. I may touch on what a few other people have pointed out, so forgive me if I sound like an echo:
1) Setlists. PJ do not have fixed setlists. Back in '03 they played three consecutive shows in Boston and only repeated one song - and that only because the audience kept yelling for it. When I saw them on Saturday I was quite surprised that they played Alive, because they'd played it the night before. They're that varied.
2) Stage production - as Headache said, there is none. Just lights and screens so people in the back can see the band better. Think U2 circa UF Tour.
3) Musicality - this is the big one, and the heart of the difference between the bands. First off, Pearl Jam jam. It's not real jamming, like jazz (no rock groups do that), but there are extended instrumental sections that change from night to night. The lead guitarist never plays the same solo twice (he's the antithesis of Edge). Pearl Jam are, from top to bottom, more musically sophisticated than U2 - but their sound is not as organic. And Eddie Vedder, as a singer, presents a fascinating counterpart to Bono: he doesn't have the range, or the stylistic versatility, but his voice is a miracle of nature. Bono's voice these days is a hit-or-miss affair. Ed's isn't. His power now, in his early forties, is undiminished from what it was fifteen years ago. And he smokes at least as much as Bono
My impression of the PJ show was somewhat corrupted by the fact that the crowd in my section was, to put it mildly, bad - I have a thread about this somewhere. But I enjoyed U2 far more. I have several friends who also saw U2 and PJ in the same week, and among them, opinion is evenly split as to who put on the better show.
What it comes down to, for me, is that the differences between U2 and PJ in concert are exactly the same as the differences between them as bands in general. PJ are more consummate musicians. They are also less versatile and less... well, mystical. What U2 does is unconventional in almost every way - never have four people of such unremarkable technical skill created such a body of music. Pearl Jam are brilliant, even sublime, and if I were to compile a playlist of the best live performances I have ever heard, they would be the only band besides U2 to make the list. (I am thinking of Black from Benaroya Hall, 22-10-03, among other things.)
If conventional musicianship is what appeals to you most, you will prefer Pearl Jam. If something else matters more - something too big to describe here - you may prefer U2.
Having said all this, I am still ecstatic over the Sydney "U-Jam" performance. Oh, to see more of that...