NEW YORK (AP) _ By coincidence, off-Broadway currently is home to a pair of new plays celebrating two different groups of formidable women.

''From the Mississippi Delta,'' on view at the Circle in the Square Downtown, looks at the black heroines of the Deep South, from the 1940s into the mid-1980s.

''A Piece of My Heart,'' a Manhattan Theatre Club production at the Union Square Theatre, examines what the war in Vietnam did to the American women who were there, as nurses, Red Cross volunteers, military personnel and even as entertainers.

Both plays are works of story theater, but ''From the Mississippi Delta,'' written by Dr. Endesha Ida Mae Holland, is a much more satisfying evening. In fact, it's exhilarating, taking on the joyous spirit of a religious revival meeting before it reaches its high-spirited finale.

Holland, whose work for her university degree title is an important part of her play, tells her life story in a series of 11 episodes. Each is self- contained. Yet woven together, they form a rich, raucous and rewarding tapestry of life in a land-rich, but dirt-poor section of the country.

The stories begin with Aint Baby, a midwife of remarkable skill whose expertise earns her the title of Second Doctor Lady among the local folks. But the real focus of the evening is her daughter, Phelia, a young girl who will grow up to be Holland.

Her tales range from the shocking - an 11-year-old Phelia raped by a white man whose wife brings the little girl to him - to the bawdy when Phelia decides to become a carnival dancer with a most unique talent.

Holland's writing is strong and richly atmospheric. She populates the stage with real people. There's a Miss Rosebud Dupree, an old lady with a proprietary interest in the water meter in front of her house. The woman has a strong throwing arm, aiming bricks at any trespasser daring to step on the meter.

There is real sense of place in Holland's stories. The delta, with its ''calm, balmy days,'' according to the playwright, becomes a character itself.

All the roles, female and male alike, are played by a trio of actresses - Sybil Walker, Jacqueline Williams and Cheryl Lynn Bruce. They are astonishing, not only as actresses but as singers, performing everything from gospel to blues to freedom anthems.

The production has been directed with sweet simplicity by Jonathan Wilson. Pots of pink petunias - symbolizing an Alice Walker poem about ''Revolutionary Petunias'' - ring the stage. A minimum of props - most noticeably an old rocking chair - are effectively used. The evocative lighting by Allen Lee Hughes makes a strong contribution.

Much of the play's second half deals with Phelia's transformation by the civil rights movement and her journey north - to Minnesota - to get an education. Her battle for recognition is as heartbreaking as it is heroic, especially because we feel for the people who were part of her struggle.

In contrast, ''A Piece of My Heart'' is wan oral history, a diluted form of scrapbook theater. The six women in the play are types rather than real people. Theatergoers never really get to know them by the time the evening reaches its emotional climax, a reunion at the Vietnam Veterans Memorial in Washington.

Playwright Shirley Lauro has stitched together the war memories with precision if not insight. The facts are there, sometimes powerful enough to make for some affecting ancedotes, but the people themselves are not.

That's the big difference betweeen ''A Piece of My Heart'' and ''From the Mississippi Delta.'' And it's an important one.