After weeks of indecision, the Prince George's County Council officially decided yesterday that if there could be only one Metro line to the southern part of the county, that line should go to Rosecroft Raceway.

The 9-to-2 vote was a reversal of an earlier position taken by the council and represents a dramatic change in the long-planned routine to the area of the Metro line from Anacostia.

For years Metro maps have shown that line as running out the Suitland Parkway and terminating at Branch Avenue, just inside the Beltway. There would have been an intermediate stop at the high-employment Suitland Federal Center. Neither of those stops is included in the Rosecroft routing.

Several council members expressed the belief that the federal government would offer to fund the Branch Avenue line anyway because of the Suitland Center and that the best way to get lines to both terminals was to choose only Rosecroft, since only one choice was possible.

"If we build the Branch Avenue line first, we'll never see Rosecroft," said Councilman Gerard T. McDonough.

McDonough had been the author two weeks ago of a council proposal to build a Y-type route, with one line going to Rosecraft and the other to Branch Avenue. That proposal had never been studied to determine whether it would be economical.

Furthermore, the proposal came in the last phase of a metropolitan area-wide restudy of the entire Metro system. Hundreds of alternative routes were examined to determine such factors as ridership, construction and operating costs and feeder bus networks required.

When the regional task force endorsed a 100-mile Metro system as cost-effective last week, it told Prince Georges County, in effect, to choose one route, at least initially, so the study could be completed on schedule.

"We are forced into a position not of choosing one line over the other," McDonough said, "but in choosing which line we will build first. In no way are we abandoning either corridor."

Rosecroft had strong support from county residents along the Indian Head Highway corridor and also received lobbying support from Peter O'Malley, an important Democratic political figure in the county and an attorney for Rosecroft Raceway.

There are several large undeveloped tracts of land near the race track. Council member Francis B. Francois, who voted against the Rosecroft option, said I don't believe the willpower exists in this county government to require (high) density of development (around the track) necessary" to guarantee high ridership and low subsidies.

"This is a developer alignment meant to support intense development yet to come," he said. "I had thought the people in that end of the county did not want that kind of development."

The Rosecroft route would cross the Beltway near its interchange with St. Barnabas Road and a station is scheduled to be constructed there, with a big parking lot to attract auto commuters. There is a historic church there, and Francois has predicted that neighborhood opposition will be high.

McDonough, generally regarded as one who sometimes carries messages from County Executive Winfield M. Kelly Jr. said that "the primary factor (for Rosecroft) is ridership, not developers."

A county staff study comparing Rosecroft and Branch Aveune terminals as options said that Rosecroft would attract 1.6 million more passengers to Metro per year in 1990 than Branch Aveune, and would cost about $40 million less to build.

Francois said Branch Avenue would serve a larger area and could draw from Suitland Parkway and Pennsylvania Avenue extended as well as from the Beltway.

Council member Francis W. White joined Francois in voting against the motion. Member William Amonett first voted no, then switched his vote after the roll was called.