Two musicals, opening on successive evenings and strikingly alike in format, show how the luck of timing may dictate vital critical reaction.

"Runaways," written, composed and directed by Elizabeth Swados, whose "Nightclub Cantata" was at Arena Stage last fall, opened first. it was admired in its initial performances presented by Joseph Papp's New York Shakespeare Festival at the Public Threater, and then was moved uptown general hosannas for the Saturday night opening, includingJack Kroll's Newsweek declaration that "Runaways' is far and away the best misivalof the season."

"Working," created by Stephen Schwartz and many collaborators from Studs Terkel's remarkable book, finally opened the next night at the 46th Street after an introduction at Chicago's Goodman Threater and several months of re-working in New York, where the grapevine pronounced disaster. There was a curious restrain in some of the notices and imminent closing was rumoured.

In my view, "Runaways" is a dispointing one-note, overrated bore and "Working" is the most original musical I've seen in too long, superbly performed and downright exciting. I doubt that the public is going to keep "Runaways" helty once word of its monotony spreads, and I'm confident that, given time, the public will turn "Working" into the success it so honorably merits.

The musicals are alike in that neither has a story line and that both are formed by a string of characterizations set to music and movement. (One hardly can call the jumping up and down in "Runaway" choreography, though Swados has that among her five Tony nominations of "A Chorus Line."

In "Runaways" we meet young people who tell us why they've left home. Inevitably, their reasons are alike, that it's their parent's fault. The tone is self-pitying, ego-centered and repetitious, which is understandable in theyoung but hardly interesting. Afterthe first few characters expound, the evenings is one long whine.

In "Working" we meet an astonishing array of youths and adults: a steelworker, a parking-lot attendant, an editor, a secretary, a stonemason, a call girl, a seaman, a cleaning woman, a bus driver, phone operators, a newspaper copyboy.

IN a striking range of songs we learn why these individuals work at these jobs. But they waht recognitions for jobs well done or at least done with grace or humor. There is pride and gallantry here, and the cumulative sensation is dynamic and upbeat.

Both musicals extend the creative collaboration which is essential in theater and films. Swados interviewed hundreds of adolescents, using the material for extensive improviastions by some of thewm or by young, inexperinced players. Some of these performers were replaced professionals for the Broadway move.

Terkel's book, "Working," stemmed from taped interviews he made in the Chicago area ove r several years. Schwartz conceived the notion of a revue form for songs and dances written by such exprienced creators as Micki Grant, Cragi Carnelia, Mary Rodgers, Susan Birkenhead, James Taylor and himself.

"Working's" performers are complete professionals, including Rex Everhart, Patti LuPone, Susan Bigelow, BObo Lewis and Arny Freeman.

"Runnaways" enjoys a slightly fuller production than it had on Astor Place. "Working" has been most elaborately staged with scenic projections and sliding floor panels. These visual excitements ballooned the budget to a reported $1.2 million, which is why Arena Stage backed away from its plan to follow the Goodman performances. Given Marjorie Slainman's array of costuming details, I suspect "Working" would have made its profound effect, though I don't deny that the staging details do add to the immensely professional atmosphere of the whole.

By professionalism I mean skills in the varied disciplines that create theatrical wholes. Somehow, professionalism is now being scorned by writers on the performing arts, there seeming to be a notion that spontaneity, erratic as it can be , is superior to controlled craft. In a sense this is an outcropping of the elitist vs. populist argument. This strikes me as asinine, for though professionalism certainly can go wrong, the likelihood of amateurism going wrong is far greater and the results of that can be either embarrassment, boredom or both.

Swados, who makes strong claims to modesty, has had the assurance to stick by her principle of theme with simplicity. But her disclaimer is made with something close to a flourish. Her results are not, after all, very different from what an amalgam of "Godspell," "The Me Nobody Knows " and "Hair" might be with assists from the police blotters and Dial-a-Counselor. What she cannot evade is repetition, reiteration, juvenile attitudinising and selfpity.

One leaves "Runaways" with a blurred vision of its many performers, but takes from "Working" a whole gallery of vivid individuals, most of them from performers doing several roles. To the names already mentioned I must add Matt Landers for his firemanand David Patrick Kelly for his hangloose addict recalling his copy-boy career, a dazzling creation.

All 17 "Working's" performers are remarkable for their polished resourcefulness, individually and collectively forming a rich array of memorable characters.

Above all, Terkel's collaborators have been true to his stirring slice of American, transferring from his tapes flesh and blood of spirited individuality, assurance and pride.

Should "Working" falter before the word gets around, a remarkable musical could vanish. That the word will be good was indicated by the thoughtful, gripped audience of which I was a part. Workers themselves, they recognized this for unique, provocative inspiration to us all.

"Working" is a thrilling and positive musical.