Happy Birthday on Sunday to the grand old men of musical theatre, and I'm not talking Rodgers and Hammerstein. It's almost freakish that Andrew Lloyd Webber and Stephen Sondheim, the transatlantic giants of the genre, share the same birth date, 22 March, though ALW is a mere stripling at 67 years, Sondheim a justly venerated 85.

Both composers, both geniuses in their own individual ways, tread warily and respectfully around each other, but their supporters, mostly on the Sondheim side, are like rival fans of the two Manchester football teams: vociferous, bitchy and unaccommodating. Lloyd Webber, in their eyes, always wants to please the public whereas Sondheim wants to change an art form. Guess who most critics prefer.

The truth is they both seek popular approval, of course, and they have both been revolutionary innovators. Lloyd Webber's two major musicals written with lyrics by Tim Rice, Jesus Christ Superstar and Evita, will remain in the repertoire for as long as Puccini's Tosca and Franz Lehar's The Merry Widow and, who knows, Sondheim's Sweeney Todd. Cats and Starlight Express are brilliant eclectic vaudevilles, exploring every kind of rock musical and operatic style in effervescent pastiche, while Sondheim's best shows - my favourites (not necessarily the same thing) are Company, A Little Night Music and Into the Woods - reinvent the genre as urban satire, fairy-tale, operetta; and of course he rewrites his own brilliant lyrics.

Although the first productions of Company and Night Music each ran a year here, neither was a commercial smash, nor was (or is) Assassins, for all its mordant vitality. ALW's Phantom of the Opera runs for ever because it touches a nerve about the music of seduction, the agency of love and the splendour of its rock romantic expression; and the music is on a constant switchback between that romance and the heart of darkness in the Phantom's labyrinth, while Sondheim does a similar thing in combining surface atmosphere with ghoulish revelation in Sweeney Todd.

'One problem they share is that there is no-one else remotely as good as either of them'

Both write "proper" music to put it mildly. The first half of the finally disappointing Stephen Ward contained some of Lloyd Webber's best continuity, or underscoring, while Sondheim's Passion, which I can live without, has some of his finest arioso and harmonic moments. If there's a musical context for Lloyd Webber's inspiration you find it in Prokofiev as well as Puccini - it's worth remembering that when Dmitri Shostakovich, arguably the greatest composer of the 20th century, saw Superstar in London shortly before he died (twice, on successive evenings) he lamented that he could not have written something similar himself, admiring particularly the writing of a core rock band orchestration overlain with full symphonic strings, brass and woodwind. And Sondheim hails from the greatest days of the Broadway musical and indeed wrote lyrics for three of them - West Side Story, Gypsy and (music as well) A Funny Thing Happened on the Way to the Forum.

Both writers owe a lot to Cameron Mackintosh. Mackintosh produced Cats and Phantom and Side By Side By Sondheim, the 1976 cabaret that really put Sondheim on the map in this country. And when Cameron celebrated his 30 years in show business in 1998, ALW and "Steve" (as I called him by the end of our one and only chance encounter in a London pub, when I had the privilege of buying him two very large gin and limes) collaborated for the first and only time at a royal gala in the producer's honour.

On film at that gala, they shared the same piano and an item (devised by Sondheim) which wedded the tunes of "Send in the Clowns" and "Music of the Night": "Isn't he rich? Isn't he square? Isn't he working the room, somewhere out there? Send in the crowds... Acts on his whims, took a big chance, seeing his anagram said: Cameron, Romance. He went to France. Send in the crowds." The tune shifted to 4/4 time: "Night time falling, Cameron keeps calling. Posing questions, questions with suggestions... suddenly appearing, always interfering; but here we are, and cheering as we might, the man who flogs the music of tonight."

About the only other thing they have in common is having written an Oscar-winning film song for Madonna (ALW in Evita, with Tim Rice, and Sondheim in Dick Tracy), and both are struggling to adjust to the changing musical theatre scene. One problem they share is that there is no-one else remotely as good as either of them, nor do they have the benefit of a great musical theatre producer like Mackintosh, last of a breed, I reckon. ALW's influence has been immense in terms of the industry and side issues like sound systems and orchestration, while Sondheim has unwittingly created a more baleful legacy in his imitators and American musical theatre writers who simply can't get out from under his skin, or his shadow.

Still, with two Sweeney Todds coming up - one at the ENO with Bryn Terfel and Emma Thompson (semi-staged, for 13 performances only, hardly an auspicious launch for the supposedly money-making ENO deal with Michael Grade and Michael Linnitt), one in a pop-up pie and eel shop on Shaftesbury Avenue, cheekily gazumping ENO on Cameron's patch - and Gypsy soon to storm the Savoy, Steve ain't going away... nor is ALW, though I'm not the only one to worry about space songs for Sarah Brightman, his muse and inspiration on Phantom, and his unlikely-sounding collaboration with Julian "Downton Abbey" Fellowes - the Downton chief location, Highclere Castle, is handily close to ALW's country pile on the edge of Watership Down - on Jack Black's School of Rock.