Six touring U.S. congressmen led by House Budget Committee Chairman William H. Gray III (D-Pa.) heard opposing viewpoints within South Africa's nonwhite political community today when they met in Cape Town with the foremost moderate, Chief Gatsha Buthelezi, and a leading radical, Allan Boesak.

The congressmen, who have said that they are here to assess the effects of limited American sanctions, heard Buthelezi describe attempts to cripple the country's economy as "foolhardy." Boesak, in contrast, is a known supporter of stronger sanctions and has been charged under security laws for advocating the pullout of foreign investors.

Gray described the meetings as useful.

Buthelezi, tribal leader of the 6 million Zulus, the biggest ethnic group within South Africa's 21 million black majority, is involved in a bitter war of words with the outlawed African National Congress and other African nationalist groups, notably the United Democratic Front.

Boesak is a founder of the Democratic Front and a key figure in its policy planning.

Buthelezi released a statement made to the congressmen in which he reflected this conflict, appealing to them not to support radical forces that he said would destroy democracy in South Africa.

Western governments "should not support that in our society which they would not tolerate in their own," Buthelezi said.

In another oblique reference to those socialist-led opposition groups, Buthelezi said that although socialist ideals were attractive to many black South Africans, it had to be recognized that economic growth and the betterment of black living conditions depended on South Africa's integration with western industrial nations.

Boesak took the congressmen on a tour of the packed Crossroads squatter camp outside Cape Town, where crowds greeted him with black-power salutes and slogans praising leaders of the ANC and United Democratic Front.

Tonight, the congressmen accompanied Boesak to a church service in a mixed-race ghetto outside Cape Town. He is a clergyman in the Dutch Reformed Church and is president of the World Alliance of Reformed Churches.

Walter Fauntroy, the nonvoting congressional delegate of the District of Columbia, and Gray -- who are Baptist ministers -- preached sermons to the congregation of 400.

Here in Johannesburg, lawyers completed their arguments in a provincial Supreme Court action in which Winnie Mandela, wife of imprisoned African National Congress leader Nelson Mandela, is challenging the validity of a new restriction order that resulted in her being forcibly removed from her Soweto home last month and arrested twice when she tried to return to it.

Judge Louis J. le Grange said he hoped to give his verdict next week.

Johann Smit, counsel for the government, argued that the government minister who issued the new restriction order Dec. 21 did not need to give reasons, as Winnie Mandela claims he should have done, because the new order effectively relaxed an earlier restriction.

Mandela's lawyer, Sidney Kentridge, argued yesterday that security laws allowing the minister of law and order to detain, silence or banish people without recourse to the courts still required him to furnish at least rudimentary reasons for doing so.