Alton Miller, the managing director of the Washington Ballet, will leave Oct. 1 to become general director of the Chicago City Ballet, a 3-year-old company founded and directed by the former New York City Ballet prima ballerina, Maria Tallchief.

The jobs are virtually the same--responsibility for fundraising, bookings and stage production. So is the salary, which Miller will not reveal.

"It's not a money choice," says Miller, a native Washingtonian. "It's a heart choice. You know, I'm almost too comfortable where I am. I love the dancers. I'm going to be 40 years old July 17. You know you can get too cozy too fast too young, and that's what I am. That's not a primary reason for making the move. But it's a factor. It's a challenge at a time when the Washington Ballet is in a different kind of consolidation period."

Miller has been with the Washington Ballet five years. During that time, the company has flourished. Its resident choreographer, Choo San Goh, has won international acclaim, and several of its dancers have won international prizes.

In 1981, the troupe sent dancer Amanda McKerrow (now with the American Ballet Theatre) to the prestigious International Ballet Competition, where she became the first American to win a gold medal. Last year, dancer Bonnie Moore won first prize in her category at the Prix de Lausanne competition in Lausanne, Switzerland. In the last 12 months, the company has toured extensively--the Spoleto Festival, France, Germany, Luxembourg and, last December, Singapore.

Mary Day, the noted founder of the Washington School of the Ballet and the artistic director of the Washington Ballet, is currently enroute to Varna, Bulgaria, to enter two of her dancers-- Moore and John Godding--in the same competition McKerrow won in Moscow.

"The Washington Ballet has had five very exciting years of tremendous growth, breathtaking growth," says Miller. "A really excellent board of directors has taken shape, really from scratch . . . a combination of fast artistic growth and a board of directors really in a position to take responsibility for the destiny of the ballet suggests that next year will be a consolidation year, a plateau year. That's normal and good.

"Now, here comes the Chicago City Ballet kind of on the verge of a full artistic flowering, and lacking--as far as I can tell--a general director. I think it's a marvelous opportunity to take all the wonderful lessons I've learned in Washington into a new scene just fraught with potential."

When Miller was approached this spring about the Chicago post, he had just rejected offers from two other ballets--the Pacific Northwest in Seattle and the Ohio Ballet in Akron. "I told them I was flattered but I had no intention of leaving the Washington Ballet," he says, "but it started me thinking."

He met once in Chicago with Tallchief, one of the late George Balanchine's star dancers as well as one of his former wives.

"She really has a vision and style," says Miller. "I'm really looking forward to the chemistry of working with her ."

Miller has never seen the Chicago company perform.

"I've talked to people who've seen them," he says. "They characterize it as a young company . . . with promise. They make a point of fidelity to the essence of George Balanchine."

Suzanne Farrell, of the New York City Ballet, sometimes dances with the Chicago ballet, and her husband, Paul Mejia, also a dancer, is resident choreographer with the company.

Miller says it would be difficult to compare the Washington and Chicago companies. "There's a profound dedication to the Balanchine esthetic at the Chicago City Ballet," he says. "The Washington Ballet's emphasis is more on the exotic, contemporary choreography of Choo San Goh.

"In some respects, the Chicago City Ballet is ahead of the Washington Ballet. Routinely, the ballet gets $100,000 from the City of Chicago. Need I say more? No dance company has ever heard of getting $100,000 from the city government of Washington."

The Chicago ballet is scheduled to perform in Newport, R.I., in the next few weeks and then go on its first European tour. "I was asked if I wanted to go," says Miller, "but that's a little short notice . . . I will make six weekend trips to Chicago to outline an agenda."

However, there is still much unfinished business at the Washington Ballet, Miller says. "My first priority will have to be the Washington Ballet this summer and early fall," he says. "I've been putting together a two-month tour of the Orient which I intend to see through." He will miss the actual tour, scheduled to begin in mid-January.

When Miller leaves, so will his wife, Cindy Bandle, the ballet publicist.

Aldus Chapin, president of the board of the Washington Ballet, said no plans have been made to replace Miller. "It wasn't really sort of official until yesterday Tuesday . We have a planning committee meeting tonight Wednesday and I guess we'll be discussing it. We were just hoping Al would stay. We've done everything we could to keep him."