THOUGH MOST of the voter attention understandably has been focused on the Democratic mayoral primary, the choice of the No. 2 leader in city hall over the next four years - the D.C. council chairman, who serves as head of the legislative branch - is very important. We say that because there are fundamental differences between the candidates for this office - Arrington Dixon, the Rev. John G. Martin and Douglas E. Moore - that lead us without hesitation to recommend the election of Mr. Dixon.

The least familiar of the candidates is surely Mr. Martin, a Baptist minister who has been active in various community activities over the years and who has siad he is running because he "was shocked to hear at the Baptist Ministers Conference that the choice for council chairman was a choice to pick the lesser of two evils. The lesser of two evils is still evil." Were that an accurate assessment of the differences between Mr. Dixon and Mr. Moore - and were Mr. Martin able to present some more concrete idea of his own agenda for the city - his candidacy might offere some real alternative.

But it boils down to a race between Mr. Dixon and Mr. Moore - and the contrast in their records on the council tells the story. Unlike some political figures who seem to have grown in stature and respect as the city's franchise has gradually expanded, Douglas Moore has, in our view, been a great disappointment; after winning an impressive victory four years ago, he has accomplished amazingly little in his years as a member of the council. And quite aside from those well-publicized incidents in which has judgment has come into question, he has failed as a legislator: There has not been one bill he can rightfully claim to have introduced and seen through to enactment. Instead, Mr. Moore has been content in the main to rely on his past fame as local activist for the rights of the poor, and to underscore his defective work record by making reckless statements around the city about the other members of the council and about various whole classes of residents.

Mr. Dixon, for his part, has been a most serious and industrious member of the council, one who as tackled many important legislative questions that have not always made headlines: improvements in voter-registration procedures, changes in the divorce law, laws to improve government services to the aging and to the Latino community and a complicated set of detailed proposals to improve the city government personnel rules - changes that could make it much easier for the leadership at city hall to replace incompetent department heads and other top-level officials. Mr. Dixon also has demonstrated an ability to work well with fellow council members - while Mr. Moore's inattention to legislative matters contributed to his loss of the chairmanship of the council's powerful budget committee.

Even beyond the strict jurisdiction of the D.C. council - to regional questions involving participation in decisions by the area's Council of Governments - Mr. Dixon has earned respect as the group's chairman, while Mr. Moore talks against reliance on the suburbs for help in jobs programs, about how the city should declare ownership of National Airport and about how the city has been "rooked on the subway payments." That kind of divisive talk, coupled with a miserable legislative record, is not what proud residents of this community deserve from the No. 2 position in city hall. Voters who demand more, who look for a serious, steady and hard-working legislator, will find Arrington Dixon a vastly superior candidate for the office.