Britney bombs, Robbie rocks

The reason why Britney Spears is so successful is that she's such excellent value for money: you get two separate people for the price of one. In photo shoots and in her videos, Spears is a soft-porn star who would make Madonna blush. But when she's interviewed, Spears is a fresh-faced small-town gal who won't shut up about her virginity and her Christianity. The virgin/whore dichotomy has never been embodied so neatly by a celebrity before, and it's this which makes her fascinating. And, arguably, dangerous. By dividing herself into two entities, Spears licenses the most sordid Lolita fantasies. Moreover, she puts those fantasies on screen, allowing potential Humbert Humberts a line of reasoning: however innocent and prim a schoolgirl might appear, she can't appear more innocent and prim than Britney - and she makes videos in which she struts around in Emma Peel catsuits and begs her baby to hit her.

The reason why Britney Spears is so successful is that she's such excellent value for money: you get two separate people for the price of one. In photo shoots and in her videos, Spears is a soft-porn star who would make Madonna blush. But when she's interviewed, Spears is a fresh-faced small-town gal who won't shut up about her virginity and her Christianity. The virgin/whore dichotomy has never been embodied so neatly by a celebrity before, and it's this which makes her fascinating. And, arguably, dangerous. By dividing herself into two entities, Spears licenses the most sordid Lolita fantasies. Moreover, she puts those fantasies on screen, allowing potential Humbert Humberts a line of reasoning: however innocent and prim a schoolgirl might appear, she can't appear more innocent and prim than Britney - and she makes videos in which she struts around in Emma Peel catsuits and begs her baby to hit her.

This sort of theorising may not have much to do with Wednesday's concert at Wembley Arena, but Spears is more interesting in the abstract than she is in reality. On the British leg of her first world tour, she was suddenly no longer an endlessly deconstructed cultural phenomenon; she was a teenage performer of pop music for children.

To be fair, the show's producers threw everything they had at it. In front of a video screen and a raised metal walkway, a squad of dancers launched themselves into their jerky, post-Jacko choreography, and a band blasted out blaring arrangements of the music. Flames roared, confetti rained, and somewhere in amongst it all, Spears zipped through a fashion show's worth of costumes. The result was a confused spectacle with more money than sense behind it, each new special effect an effort to distract us from the absence of original ideas.

Worse, both the songs and Spears herself were overwhelmed by the audiovisual barrage. From The Bottom Of My Broken Heart was staged as an MTV Unplugged ballad: Spears sat on a stool to sing it, while the bandleader strummed an acoustic guitar beside her. It was just a pity that the backing singers, synth player, drummer and bassist ruined the effect, and that Spears' voice was barely audible. Assuming it was audible at all. When she greeted the crowd in between songs, she sounded so bunged-up and croaky she could have starred in a Lockets ad, so I have my suspicions about her singing.

If Spears was struggling through a cold - and in those skimpy outfits, it's only to be expected - it could explain why she made such a faint impression. Or it could be that she's simply better suited to videos, when the camera can linger on her seductively made-up Princess Di eyes. When she's far away in a massive arena, in a throng of dancers, and racing from set piece to set piece without a moment to catch her breath, Spears adds a third character to her split personality: an eager young stage-school trouper who tries very hard, but doesn't quite succeed.

There was no danger of Robbie Williams being reduced to a bit-part in his own concert. The staging of his show on Tuesday was almost as extravagant as Spears' - animations, confetti bombs, waterfalls of sparks - but it was all stamped with his own inimitable cheekiness. Who else could open a show with a tape of "Robbie, Robbie!" being chanted to the earth-shaking strains of Carmina Burana? Who else would perform on a stage flanked by giant Brit Award statues? Most Robbie-ish of all was the teasing announcement just before the show began. "Robbie Williams will not be appearing tonight," it said. "The good news is that H from Steps will be replacing him." The message is loud and clear. H, like Williams, may have found fame as the jokey one in a chart-topping manufactured pop group, but can he or anyone else in the top 40 compare with Rob? Of course not.

As ever, Williams put in the dynamite performance to justify his braggadocio. His glam-pop epics rang around the arena, powered by a top-flight band, but their singer had the voice and the charisma to dominate proceedings. Wherever there was a stop, Williams pulled it out. He danced, he jumped, he flirted, and there's no doubt that the audience had a great time.

Whether Williams had such a great time is another matter. He is more or less incapable of singing a song without deflating it with a theatrical glance at his watch or an irrelevant aside: during Knutsford City Limits he suggested we talk among ourselves while he pulled a sweatshirt over his head. I ended up as distracted as he was. If he can't lose himself in what he's singing, you have to wonder whether he holds his songs, his audience and even himself in contempt. In that light, those Brit statues and the Carmina Burana intro raise similar questions. Does he believe that the acclaim he's been accorded is ridiculous? If so, why does he feel the need to slag off other performers? And when he does his deliberately camp, thespy bows, does he believe he's worthy of the applause or not? I doubt that Williams himself is entirely sure of what's lies beneath all the layers of irony. In the midst of an exultant concert, he still comes across as a confused, unhappy young man being strangled by a pair of ever-tightening inverted commas.

One of the only songs he played straight on Tuesday was She's The One, and that's the one song on his albums that wasn't written by him and Guy Chambers, but by World Party's Karl Wallinger. Surely, if Williams doesn't think his own compositions are worthy of the same respect, he should keep working on them until they are. If he ever let himself be sincere, he might just move us and thrill us, not just entertain us.

Britney Spears: London Arena, E14 (020 7538 1212) 14 & 15 Nov. Robbie Williams: SECC, Glasgow (0141 287 7777) Mon to Wed; Newcastle Arena (0191 401 8000) Fri & Sat