IN ADDITION TO the presidential and congressional contests and Virginia constitutional questions that will come before them next Tuesday, Arlington County voters will have an important local election to decide, as well as a ballot question on county financing to answer. The contest is for a seat on the five-member Arlington County Board, and it comes at a critical time in the board's deliberations, since many significant development decisions -- affecting both the aesthetic and social makeup of the county in the future -- are in offing.

Since Republican-backed members now hold three of the five board seats, the GOP majority will not change, regarless of whether Democrat John G. Milliken or Republican-endorsed independent S. J. "Sim" Pace wins. What is at stake, as Mr. Milliken has pointed out in a vigorous campaign calling for a balanced approach to housing and development questions, is a possible swing vote on many of the board's future decisions involving high-rise office buildings, conversions of rental units to condominiums and construction along Metro subway corridors.

On these and many other aspects of county government policy, Mr. Milliken's superior experience in public service and his detailed position papers make him by far the stronger candidate. An attorney and former executive assistant to Rep. Joseph L. Fisher (who strongly endorses him), Mr. Milliken has been active in civic and community affairs; he has served as a member of the county's transportation commission, chairman of the County Democratic Committee, member of the Metro Study Task Force of the Washington Center for Metropolitan Studies and chairman of the 1980 Cencus Committee for the county.

Mr. Pace, a vice president for systems development at Blue Cross and Blue Shield of Washington, emphasizes his background in financial management. But Mr. Milliken's campaign platform is much broader and more detailed. On housing, for example, he supports incentives that would encourage owners of existing rental properties to preserve them, an increase in the amount of available housing through requirements that residential construction be included in Metro corridor development, and improvements in tax and rent assistance programs. With so many important decisions on zoning, land use, tax policy and housing coming up, Arlington voters should welcome the opportunity to enlist the background and balance that John Milliken can bring to the county board.

The lone county ballot question has to do with improvement of water supply lines. Voters are being asked to approve a plan to use $6 million left from a bond issue that was approved three years ago. This is not a proposal for a brand new bond issue, but merely a sensible request to redirect money that was originally designated for construction of water storage facilities to an improved water supply-line system -- which deserves a YES.