TWO MUSICALS, both the products of distinguished collaborations, have opened almost simultaneously -- "Carmelina" at the Kennedy Center Opera House, and "Sweeney Todd" at Broadway's Leon Uris

But the products could not be more unlike each other. The first represents a late blooming of a musical tradition that flowered in the 1940s and 1950s; the second intends to break through traditional boundaries.

I suspect that critics, on the whole, will deplore the cheerfully square concepts of "Carmelina" and enthusiastically acclaim the brilliant innovations of "Sweeney Todd."

I also suspect that audiences will react in reverse. They will relish the melodies, humors and humanity of "Carmelina" and veer away from the ingenuities, conceits and harshness of "Sweeney Todd."

Neither is perfect, but I relish them both for what they are, and what they reveal not merely about the state of American musical theater but also about the comforts of the familiar and the exhilaration of the unfamiliar.

Inspired by "Buona Sera, Mrs. Campbell," an uninspired Lollobrigida movie of 1969, "Carmelina" has a book by Alan Jay Lerner, who also wrote "My Fair Lady," and Joseph Stein, librettist of "Fiddler on the Roof." Burton Lane ("Finian's Rainbow" and "On a Clear Day") wrote the music. All its production credits -- direction by Jose Ferrer, choreography by Peter Gennaro, sets by Oliver Smith, costumes by Donald Brooks, lighting by Feder -- are by masters. Georgia Brown, who sang "As Long as He Needs Me" in "Olibrt!" and Cesare Siepi, the Metropolitan's great Boris and Don Giovanni, sing the leads, and gloriously.

Brown is an Italian mother, who, in one busy World War II month, had comforting intimacies with three American soldiers. Each believing he fathered a daughter, all have since contributed monthly checks to make Carmelina rich and respectable. Now they return for a regimental reunion and discover themselves "stockholders" in the same family.

Frankly traditional in staging -- even to reviving "follow spots," those circles of light that follow principals as they move across a darkened stage -- "Carmelina" is an honest throwback and will make theatergoers very happy indeed.

What matters most are the lovely melodies; and Lerner's lyrics are deft and the voices could not be finer. There is neat humor in the three men whose reunion reunites them so confusingly with Carmelina: John Michael King, Howard Ross and Gordon Ramsey. They have a giddy trio, "The Image of Me" and a lovely melody, "One More Walk Around the Garden."

The songs are so good I wish there were more. Why not one, say, for Virginia Martin, so hilarious in "How to Succeed" and here playing the wife of the sweetly oafish Ramsey. Maybe the action could start more swiftly, though Brown's expository "Why Him?" asserts her situation perfectly. Perhaps more dancing from Lane's evocative, Neapolitan rhythms? Act II is dangerously brief.

At all events, "Carmelina" is true to itself, the kind of romantic, melodic, gently humorous musical which audiences haven't had a crack at for too long.

"Sweeney Todd" is so brilliantly, theatrically innovative, so ingeniously staged and so striking in Angela Lansbury's fruitily comic performance that one regrets, as Act II takes hold, that it trades its humor for pretention, permitting tongue-in-cheek to veer into a lecture on injustice.

Billed as a "musical thriller," "Sweeney Todd" is derived from a widely popular 19th-century melodrama subtitled "The Demon Barber of Fleet Street." Librettist Hugh Wheeler adapts a politically tilted 1973 adaptation by Christopher Rand.

Sweeney's life has been ruined by a judge who, coveting Todd's wife, sent him off to prison. When Todd returns 15 years later, he finds no wife -- and the same judge planning to marry his stepdaughter. Todd aims for vengeance, but he realizes it takes a series of murders by happenstance to get the judge's throat where Todd's razor wants it.

Sweeney Todd's willing aide is his landlady, Mrs. Lovett, who manufactures "The Worst Meat Pies in London." She will dispose of Todd's bodies by turning them into meat pies. With "A Little Priest," a patter song of dazzling verbosity, Act I ends at the pinnacle of sophisticated whimsy, a black joke of fiendish ingenuity.

How to describe Lansbury's raucous Mrs. Lovett? She enters smashing through a door, tackling her dough with venomous elan, a Julia Child with Lady MacBeth yearnings. Her hair is frizzed up into two diabolic horns atop her head, her diction never misses a syllable of Sondheim's rapid-fire rhymes and her voice reaches tellingly for his most perverse notes. Sondheim is not a composer to merely begin a melody and then bring it to a climax. He purposefully shies away from that, disdaining musical promises. Lansbury's is a performance worth traveling leagues to see and likely will bring her her fourth Tony win.

This comic edge, opposite Lou Cariou's strongly acted Todd, yields in Act II to solemnities. Sondheim, Wheeler and Prince are determined to lecture us on the obvious cruelties of the industrial revolution. Who needs it? Anyway, Brecht and the Berliners have told us. This solemnity is the crack in Sweeney Todd's crystal.

Prince's production is breathtaking. Designer Eugene Lee brought a mid-19th-century iron foundry down from Connecticut onto the huge Uris stage before a backdrop illuminating the London of the period. The street people and their dress recall Richard Foreman's "Three Penny Opera" at Lincoln Center. Never is there a lifeless instant.

Sondheim's music and lyrics become schematically operatic. As the mood, along with the Brechtian staging, increasingly darkens, the ballad that serves to frame the story achieves the effect of a Kurt Weill oratorio. Yes, the score is a Sondheim breakthrough, but it still seems to have strayed from the needs of a mere 19th-century thriller.

There is a swirl of subsidiary characters, which always helps musicals. Merle Louise is the vanished wife, Edmund Lyndeck as the judge, Victor Garber as Todd's sailor friend, Joaquin Romaguera as a rival barber and Ken Jennings as the latter's superbly imagined assistant. All sing and act beautifully.

What this awesome, complex work has not achieved is unity of definition. Its attempt to be both satire and agitprop it will leave audiences behind while pushing our musical stage lightyears into the future. Other Newcomers

Two other new musicals, "They're Playing Our Song" and "The Umbrellas of Cherbourg," are relatively trivial.

The first, at New York's Imperial Theater, is a two-character story by Neil Simon to music by "A Chorus Line's" Marvin Hamlisch and lyrics by Carole Bayer Sager, a story said to suggest the relationship of this composer and his wordsmith.

Anyway, girl meets boy and never loses boy but has one boring time shaking a former one, Leon, whom we never meet but is a drag at a distance. Lucie Arnaz, everybody's girl-next-door, is gifted and likable, but the true surprise is Robert Klein, free at last from his talk show persona.He's witty with a line and vastly helpful to his costar.

The device of three added voices for both characters is pointless, and not even Robert Moore's polished staging makes them viable. But the houses are packed -- and delighted.

"The Umbrellas of Cherbourg," adapted from Michel Legrand's 1960s musical, threatens to bury the audience alive in silly putty. At Joseph Paap's Public Theater Martinson Hall, Andre Serban's staging and the production scheme use simplicity for style. But the sentimentality reeks, and such lines as "The oil in my car needs changing" (sung in the hero's garage) make the stomach twitch.