Katy Perry, Water Rats, London<br/>Madonna, Wembley Stadium, London

As this year's controversialist sparked mayhem at a pub gig, across town the original material girl was sounding, dare we say it, old

As any progressive-minded linguist will agree, meaning follows usage, not the other way around.

For example, I'm old enough to remember even older people saying "Gay used to mean happy...", and always blithely dismissed their objections. But, for a number of years, American youth has been using "gay" to mean "lame" or "weak", drawing a direct connection between those qualities and homosexuality. I'm standing in the way of this tide with a big "No Pasaran" placard, blocking the path of its latest instigator, Katy Perry.

Perry is a 23-year-old from California whose parents were pastors, and whose first album at the age of 16 was a Christian gospel record. It's worth holding this in mind when pondering the intentions behind her breakthrough song, "UR So Gay". The clumsy chorus goes "You're so gay and you don't even like boys", so it isn't directly an attack on homosexuality – but it is certainly a rancid piece of work, reinforcing a conservative ideal of masculinity with its attack on an indie/emo guy whose sins include not eating meat, being pale and skinny, listening to Mozart and reading Hemingway and, lord help us, wearing make-up.

Perry is in the UK to honour a tiny pub gig that was presumably booked before she went mega. It's surely the first time a tour bus this big has been parked on Gray's Inn Road, and the local chav kids have gathered outside the Water Rats to peer at the live feed on the television monitor inside.

She works a crowd that's 50 per cent fan club, 50 per cent cynical hacks like me, conducting the overhead handclaps. Capitol have wheeled out the big guns to hone the commercial viability of Perry's sound, which mainly consists of mainstream pop-rock in a Pink meets Kelly Clarkson meets the Go-Go's vein.

Her worldwide smash hit "I Kissed a Girl" is the inevitable finale. One doesn't know whether to be insulted by the idea that girl-on-girl action is still considered shocking, or irritated by the spectacle of faux-lez girls who are just, to borrow a phrase from Culture Club, kissing to be clever. On the other hand, Perry is addressing a culture (Middle America) whose idea of a woman gone a little wild is Shania Twain wearing men's shirts, so even baby-steps have to be applauded.

It's the cause for mild mayhem tonight, and the TV camera is ripped from the ceiling midway through, meaning that the chavs' free show is over. They aren't missing much. Ultimately, Katy Perry is Sarah Silverman with a guitar around her neck: a purported controversialist who, at the end of the day, is merely doing the dirty work of the Right.

Over at Wembley, another controversialist – returning to the scene of her cone-breasted prime – is cashing in. A nifty hydraulic video screen spells out C-A-N-D-Y against a backdrop of swirling sweets, a steel staircase is wheeled forward, and at the top, sprawled on a high-backed throne with one leg slung over the chair-arm, is Madonna.

Now, there was a time when having Madonna's crotch thrust in your face was many a young man's fantasy. In 2008, however, the spectacle has gone beyond Mrs Robinson and is nudging towards the wrongness level of the granny in Little Britain who gets snogged by David Walliams. But whatever your opinion of Madonna as a human being, as a pop star respect is due. Even if it's largely for things she did when Reagan was still in power.

The show is expensive enough. She rides a vintage white Rolls-Royce on to a catwalk, flanked by dancers dressed as anything from boxers to Da Vinci Code monks. Overhead, a cylindrical video screen is used to show Kanye West's duet parts, footage of starving peasants in Rajasthan, and a film of Madonna trapped inside a lift.

But money can't buy you charm. If there's one thing you'd expect Madonna to have learned after 25 years in showbusiness it's how to rouse an audience, but aside from feeble appeals like, "Alright London, are you ready to rock the house with DJ Eric Jao?", she barely bothers. Clearly, the dazzling wow-it's-Madonna factor is supposed to be enough.

Much of the show involves showing off her gym-hardened physique, as she Cossack-dances, humps the floor, and flails her hair around like a Pan's Person. There's a boxing ring for "Die Another Day" and skipping ropes for "Into the Groove". All of which renders her already-hackneyed 20th-century clichés of erotica (leather, lace, boots, canes) even less attractive.

Down on Bobby Moore Way, drunken women with northern accents, all of them £70 lighter, trickle towards the Tube, singing the unperformed "Material Girl". They didn't get what they want. Somewhere inside the stadium, the Material Girl herself, several thousand £70s heavier, rests her biceps. She, as always, gets what she wants.