WHEN WILLIAM LLOYD SCOTT performed his greatest public service for Virginia - the announcement of his retirement from the U.S. Senate - voters in both parties had every right to the assumption that any successor would be an improvement.Widely traveled, and narrowly focused, Mr. Scott has been, in a word, an embarrassment. But the assumption that anybody would be better, while possibly still sound, has been put to a heavy test by the race between Democrat Andrew P. Miller and Republican John W. Warner. The choice, let us say right at the outset, is not a particularly attractive one. But in the course of a rough and ragged race something more than a dime's worth of difference has developed between the two - especially if you take as your benchmark of excellence, or lack thereof, the performance of Mr. Scott.

The difference is that while Mr. Miller's campaign has been a good deal less than inspirational, it has offered some promise of substantive change for the better, in the talents, energy and experience Mr. Miller would bring to the job. Mr. Warner, on the other hand, has somehow managed to cast himself almost in the image of Sen. Scott, to present himself as a worthy successor, if you will - wobbly, shallow, erratic and contradictory in his public statements about both his past actions and his current views. Not only has he offered several versions of his record on affirmative-action efforts as secretary of the Navy, but in his zeal to impress the ultra-conservative wing of the party, Mr. Warner has embraced - and then qualified his support of - economic measures such as the Kemp-Roth deept-cut approach to taxes. (Sen. Harry F. Byrd Jr., the independent whom Mr. Warner claims he would reinforce rather than cancel out with his votes, voted against Kemp-Roth).

Perhaps one should not make too much of Mr. Warner's claim that he contributed not "one penny" to the 1972 Nixon campaign, when the records show otherwise. The contributions, after all, totaled only about $6,000, which could reasonably be considered small change, and readily forgettable by a fellow who has lent his own campaign something in the neighborhood of $320,000 - after saying he would not make extensive use of his personal wealth in the general election campaign.But that is merely a sample of the sort of thing we are talking about, just one of a collection of statements by Mr Warner that, it has turned out, do not quite square with the facts. The contradictions have been more than sufficient to cause concern about how responsibly he would be able to handle the obligations of a U.S. senator.

Mr. Miller's duller, lecture-style recitation of the issues tends to camouflage his superior knowledge of the state government, his interest in the law and his skills as a legislative technician. He also has a much sharper sense of the needs and priorities of his state. On federal fiscal matters, Mr. Miller has advocated a more thoughtful approach to tax relief, noting that it would be "irresponsible" and inflationary to approve a sweeping tax reduction that is not offset by reductions in federal spending. And so it has gone throughout most of the campaign, with Mr. Warner wandering, tangle-footed, all over the lot, and Mr. Miller making a reasonably earnest effort, from time to time and not without some show of opportunism, to talk seriously about important issues.

Those who yearn to carry on the tradition of William Scott would have a high probability of doing so by voting for John Warner. Those who seek a safer, sounder course for Virginia would, in our view, be better advised to vote for Andrew Miller.