House-Senate conferees are scheduled to begin the work of reconciling their two tax bills that are $12.8 billion apart in total cost but would make a maximum difference of only $130 to the average family.

For all the differences between the measures, the primary distinction for a family of four earing $15,000 a year is that the Senate bill would cut taxes by $227, while the House bill would trim them only $97.

For a family the same size in the $25,000 bracket, the difference would be even smaller. The Senate bill would cut taxes by $372, compared with $290 in the House bill - a difference of $82.

But the bills do present some important policy choices, both for Congress and the administration, and President Carter is threatening to veto the tax legislation unless it is changed to his liking.

The president has scheduled a a.m. meeting today with the chairmen of the two congressional tax-writing committees, Sen. Russell B. Long (D. La.) and Rep. Al Uilmee (D-Ore) to make clear what he will accept.

And Treasury Secretary W. Michael Blumenthal visited Capitol Hill offices yesterday urging conferees to support administration recommendations. The conference is expected to end late tomorrow or early Sunday.

Carter wants the tax bill trimmed from the $29.1 billion in cuts passed by the Senate, and wants the bulk of the cuts given to tax payers in lower-income brackets. He also wants smaller reductions in capital gains taxes, than the bills provide.

The Senate bill contrasts with a $16.3 billion tax cut the House passed in August. The Senate added $5.8 billion in extra relief for low-income and middle-income persons. It also tripled the House-passed cuts for high-income investors.

Perhaps the major worry of the administration is the potential cost of the bill, particularly several years from now. The Senate bill would cost $35.8 billion by 1983, and possibly $100 billion with a last-minute amendment senators added at the request of Sam Nunn (D-Ga.)

Administration officials are worried that unless the bill is scaled back sharply, these extra costs could bust the budget, beginning as early as 1980, and beyond - shattering any chance of reining in the budget deficit.

Republicans, however, began a campaign yesterday to muster support for the Nunn amendment: a Democratic version of the GOP's Roth-Kemp tax cut plan.

The amendments would provide $142 billion in tax cuts for individuals between 1980 and 1983, to be parceled out automatically if the government meets specified targets for holding down federal spending.

Although the provision is expected to be scrapped by the conference committee, Sen. William V. Roth (R-Del.), a co-author of the Roth-Kemp bill, gathered GOP leaders at a news conference to pledge support for the Nunn idea.

Roth has embraced the Nunn provision as "Son of Roth-Kemp bill, co-sponsored by Rep. Jack Kemp (R-N.Y.), would automatically cut taxes by 33 per cent over the next three years, with no required spending cuts.

Besides the disputes over the total cost of the tax bill, Carter faces three major conversies in the conference today: How to distribute the cuts for individuals, what to do about capital gains, and how to preserve "reform."

TThe House bill contains $10.4 billion in tax cuts for individuals, with the bulk of the reductions going to "middle-income" taxpayers in the $17,000 to $50,000 brackets. Virtually none would go to the poor.

The Senate revamped this substantially, alloting 78.9 percent of its cuts to those in the $5,000 to $50,000 brackets, but it did so by increasing the cost of the reductions for individuals to $21.1 billion.

The problem will be whether the conferees will be able to agree on a distribution that Carter regards as equitable without increasing the cost of the bill beyond the budget.

Perhaps the knottiest problem is over capital gains taxes. Carter earlier opposed any reduction in capital gains, but agreed to accept some after the House insidted on it. The question is, how much?

The House bill would cut capital gains taxes by $1 billion by reducing the maximum rate on capital gains to 35 percent (from 49.1 percent now), with the benefits going entirely to those making $50,000 or more.

The Senate adopted a different formula, exempting 70 percent of a capital gain from taxation instead of the present 50 percent, reducing the maximum effective tax rate to 21 percent and broadening the distribution somewhat.

The conference is expected to compromise by adopting the Senate formula with a 65 or 60 percent exemption, which would pose a difficult choice for the White House. Signs are that Carter might support a 60 percent exemption.

However, the problem is also includes what the conferees do about the bill's proposed dilution of the so-called minimum tax, which was enacted in 1969 to prevent high-income investors from escaping payment of taxes entirely.

The HOuse has voted to exempt capital gains from the minimum tax, which would virtually wipe that tax out. The Senate has fashioned a new, alternative minimum tax, but Carter opposes it as a "loophole."

Moreover, the conference may be complicated by budget problems. Senate liberals, who oppose big cuts in capital gains taxes, loaded up the bill with general cuts for all individuals which will force conferees to choose between the two.

The question for "tax reform" has become how to hold on to the loophole-closing actions Congress took in 1976 - not how to add new crackdowns. Most of Carter's January "tax reform" proposals have been rejected by both houses.

The Senate added to its bill a provision that effectively would undo a key proposal in the 1976 Tax Reform Act. That 1976 provision would raise income tasks on profits heirs make from the sale of inherited property. Carter opposes this reversal.

There's one final consideration: The Senate bill would offset the impact of inflation and higher Social Security taxes for most categories of taxpayers, but the House bill would not. And there's not enough money to do it all.

The tax cuts for individuals in both bills would stem mainly from increasing the $750 personal exemption to $1.000 for each taxpayer and all dependents, raising the standard deduction and reducing tax rates.