WHATEVER EXCUSES some voters may offer this fall for not choosing between John F. Herrity and Audrey Moore for Fairfax board of supervisors chairman, the old "not-a-dime's-worth-of-difference" analysis just won't be possible. Hundreds of millions of dollars of difference stand between the two -- in the form of a business boom that has made the county one of the most attractive and prosperous commercial locations in the country as well as an area committed to coping full-time with the effects of this dramatic transformation.

Mr. Herrity, long an unabashed supporter of the economic development efforts approved by large majorities of the county boards over the years, seeks reelection. Mrs. Moore, a longtime and often lone opponent of measures to promote commercial growth and build the transportation network to support it, seeks to blame Mr. Herrity for leading a reckless pursuit of development that, she says, has produced a transportation nightmare. If there is any similarity in the two candidates' approaches to county politics, it is the generally low regard each has for the other and the stridency with which it is expressed.

Though the chairman in Fairfax County has no more power than other members of the board, the job is in effect titular head of the county government and comes with an opportunity to play a major role in dealings with the state, the Washington region, the federal government and public opinion. The constituency has undergone fascinating changes, too, in the nearly 16 years since both candidates were first elected to the board. The old farms and tobacco plantations have become subdivisions for countless young and relatively affluent families that have adopted Fairfax as their home and work place.

Have these families and places of work been the ruination of the county -- causing impossible transportation problems? Or have they contributed toward a county with a solid tax base, generally good services and a state/local/private commitment to massive transportation improvements in the years immediately ahead? The answers to these pivotal campaign questions will be forthcoming in no shortage of words from each candidate -- and many voters, including some of the new wave of community-minded business leaders, may find the level of debate disappointing, short of what a county of Fairfax's stature deserves. Until people address that longer-range challenge, their options may not thrill them; for now, the focus should be on who makes the best case for a livable county -- and who is likely to be the most realistic about sustaining it financially.