Montgomery County, which in recent years has been outpaced by Fairfax County in the intense regional competition for white-collar industry, has landed a prestigious corporate prize that eluded its Northern Virginia neighbor.

Southern Pacific Communications, a California-based subsidiary of the Southern Pacific railroad conglomerate, is expected to announce today that it will move its headquarters to a 10-acre site along the I-270 industrial corridor. The site is near Marriott Corp.'s headquarters in Bethesda.

Emphasizing his administration's interest in drumming up new business, Gov. Harry Hughes is scheduled to be present for the announcement in Rockville.

Initially, the Southern Pacific subsidiary, which expects to break ground in May of next year, will have 400 employes in Montgomery, but that number will grow "to more than 2,000" in the near future, according to sourses familiar with the company's plans.

That would make Southern Pacific Communications, which has taken on American Telephone & Telegraph Co. in the increasingly competitive telecommunications industry, one of Montgomery County's biggest private employers.

IBM has 3,500 workers in the county, and Vitro Laboratories Marriott, Geico Insurance and Sears, Roebuck and Co. all employ more than 2,000.

"We are delighted," said a spokesman for County Executive Charles W. Gilchrist. "it shows we are still a premier location for industry."

Across the river in Fairfax, which spends $700,000 a year promoting itself to industry, officials conceded defeat in what is only one round for the county.

"We sought Southern Pacific, but the company chose Montgomery," said David A. Edwards, executive director of the county's Economic Deveopment Authority. Edwards said a more attractive land-purchase deal on the Montgomery site made the difference.

In the last several years, since it adopted an aggressive promotion program, Fairfax has lured some major corporate headquarters and divisions, including Mobil Oil, which is building its domestic marketing headquarters there, Boeing Computer Services and a Sperry Rand division. Two years ago, the county landed the regional long lines headquarters for AT&T.

Montgomery, with its highly educated, well-paid labor force and choice sites on the I-270 corridor between Bethesda and the Frederick County line, only recently decided it had to go out and seek similar customers.

It plans to spend $100,000 this year on promotion.

Montgomery, Fairfax and other localities are hunting for white-collar industry because they believe it generates more revenue than the expenditures required to serve it. And offices are supposed to be more environmentally compatible than warehouses, assembly plants or heavy industry.

But some planners are beginning to question the wisdom of attracting even white-collar businesses.

Santa Clara County, Calif., for instance is considering a moratorium on new industrial development like that sought by Montgomery and Fairfax because the county is experiencing a severe housing shortage and air pollution from the traffic congestion that followed new industry to the area.

Southern Pacific Communications' main service is offering long-distance telephone service to business and government customers that it says it can do more cheaply than AT&T.

Unlike Ma Bell, the new firm does not have to charge higher rates for long distance to subsidize other services, a Southern Pacific official said.

To use the service, which covers 70 cities including Washington, a customer needs a push-button phone to place a call first to Southern Pacific's computer, then dial a six-digit identiification number and finally the number the customer is trying to reach.