Of all the pop-cultural offspring of the 1980s, none have aged as well or as publicly as Bono and Madonna. Yet, as they take their latest turns in the limelight – Bono with a return to Madison Square Garden next week with U2; Madonna with her new album “Confessions on a Dance Floor” – they find themselves in very different places. Madonna is the least relevant she’s been in ages, while Bono has reached a new apex in his public stature.

For my generation, whose sexuality dawned on them sometime in the mid to late 1980s, Madonna was the essence of sex. Our earliest fantasies were haunted by images of her bared midriff, lace, and bangle bracelets. She always seemed to be at the center of some scandal of her own creation: pretending to masturbate on stage during her Blond Ambition tour, in bondage leather for the “Express Yourself” video, nude in her notorious “Sex” book. No one was as pervasive or as outrageously perverse. But there was always the sense that her target was loftier than mere titillation and record sales. She preached freedom of expression and fought prudery in all its forms.

Bono, too, was more than his music. In the late 1980s, U2 emerged as post punk with a conscience and a big message (or messages): peace in Northern Ireland, the end of apartheid, environmentalism, you name it. At every turn the stage got bigger, and Bono’s charisma and ambition swelled to fill it. Of everyone that participated in 1985’s Live Aid concert, no one defined or was defined by it as much as Bono and U2.

One has to think that the Madonna of old would be mortified by what she has become. Once a cultural harbinger without equal, adored and mulled over by the likes of Norman Mailer, she is now writing children’s books and enthusing about squishy subjects like yoga and Kabbalah for fashion magazines. She has descended into Jewish mysticism and a mystifyingly conventional private life. It’s as if she won her war against decency, then took up with the losing side.

Maybe it was inevitable. Riding the first wave of televised music culture, she was perfectly positioned to push the boundaries of public acceptability. But she was so successful at it that she left no taboo unbroken. Her former antics would seem tame in an era of hip-hop lyrics and Paris Hilton sex tapes; there’s no room for Madonna in the world she created. The only remaining opportunity to shock was for Madonna to be utterly ordinary, which is just what she’s become as Mrs. Guy Ritchie.

Bono also has undergone a transformation in recent years, but in his case it is a return to form. The 1990s saw U2 shedding much of its moral and musical baggage, delving into electronic sounds and postmodern postures with albums like “Achtung Baby,” “Zooropa,” and “Pop.” But with 2000’s “All That You Can’t Leave Behind,” the band returned to its old earnestness and save-the-world ways. After September 11, the album gave voice to some unspoken yearning for meaning that many people were feeling. Paradoxically, the band met a public need when it stopped trying to cater to public taste. “I think U2’s often at its best when it’s very uncool,” Bono recently told Rolling Stone.

By narrowing his focus to Africa (no small ambition, that), Bono has also vastly increased his political potency. Through his policy and advocacy group DATA (Debt, AIDS, Trade, Africa), he has become a major player on issues of debt relief and foreign aid and a fixture at Davos and the most recent G-8 Summit. A recent New York Times magazine profile called him “the most politically effective figure in the recent history of popular culture.”

For both Bono and Madonna, religion has been a constant theme in their music, and is a vital element of their current identities. But here again, it comes to different ends. If Madonna’s conversion to Kabbalah seems like a sideshow – and the press tends to treat it that way – it may be because she has abused religion so much in the past. A fallen Catholic, she used to treat faith and its symbols as a seduction, a tool in the service of eroticism. As she shocked the faithful, Madonna made oversize crosses a fashion accessory in the ’80s, and her video for “Like a Prayer” included stigmata, burning crosses, and a make-out session with a black Jesus.

Bono, on the other hand, has always been reverent, if vague, about his faith. Rather than aligning himself with some fringe sect, he calls himself a “meandering Christian,” as if to appeal to the broadest possible audience. In a recent interview with Rolling Stone, he explained that the music of his biggest influences – Bob Dylan, John Lennon, Patti Smith – always sounded like liturgy to him, not songs but prayers. “‘How many roads must a man walk down?’ That wasn’t a rhetorical question to me,” he said. “It was addressed to God.”

The same can be said of Bono’s best songs. He has a talent for investing music with a whiff of the transcendent, even when he’s not singing about religion. Take, for instance, these lines from “I Still Haven’t Found What I’m Looking For”: “I believe in Kingdom Come / Then all the colors will bleed into one / Yes, I’m still running.” It has the grandeur of secular prayer without being labeled as one.

Bono’s respect for faith of all stripes has made his own a powerful asset in his political efforts. Using their common emphasis on faith, Bono has often been able to persuade the likes of President Bush, Jesse Helms, and the religious right to come over to his way of thinking.

Their latest musical efforts reflect their different outlooks. Madonna’s “Confessions on a Dance Floor” is yet another foray into thin Euro dance (this time with beats by retro-minded French DJ Stuart Price, aka Jacques Lu Cont). But worse than musical unfashionability is her lyrical narcissism. “It’s funny, I spent my whole life wanting to be talked about / I did it, just about everything to see my name in lights,” she sings on “How High.” “Was it all worth it, and how did I earn it? / Nobody’s perfect, I guess I deserve it.” Madonna continues to contemplate Madonna even when the rest of the world has ceased to.

Many of the songs on U2’s “How To Dismantle an Atomic Bomb,” on the other hand, can be read two ways: as love songs or laments for an ailing world. “Miracle Drugs,” “Love and Peace or Else,” “Crumbs From Your Table,” “Yahweh” – these are the work of a man who sees himself reflected not in the mirror but in the larger world. Bono may still “like the sound of my own voice,” as he sings midway through the album, but at least he still has plenty to say.

U2 will perform next Monday and Tuesday nights at Madison Square Garden (32nd Street and Seventh Avenue, 212-465-6741).