The Reagan administration's imminent bomber decision dramatizes the difficulty of choosing the right weapon for the right time in today's world of technological "unk-unks": unknown unknowns.

The choices before President Reagan and Defense Secretary Caspar W. Weinberger sound simple enough. They are to build no new bomber at all; to produce either the B1 or the radar-evading Stealth, or to go for broke and build both of those planes at once.

The no-bomber-at-all option will not fly, if for no other reason than that the Republican Party platform Reagan stood on called for "accelerated development and deployment of a new manned strategic penetrating bomber that will exploit the $5.5 billion already invested in the B1," which Jimmy Carter canceled, in perhaps his most controversial defense decision.

But building the B1 along risks presenting the nation in the late 1980s with a bomber the Soviets could knock down with an advanced air defense. This defense, according to departed experts from the Carter administration, will feature planes with radars that look down to pick out low-altitude invaders like the B1 and computer-controlled anti-aircraft missiles on the ground.

And then there is the cost of a gussied-up B1. Here you can get almost any figure, depending on whom you ask, going up to $200 million a bomber. The likely price tag on 100 updated B1s is one of the big junk-unks, a frustration for Air Force leaders, who for months now have been trying to nail down the contractors with everything short of railroad spikes.

No matter what the per-plane cost, the B1 would take billions out of the Air Force airplane budget shortly after the orders were placed. The Air Force in the Carter administration climbed the easy part of the cost slope, the research and development phase, but no the multibillion-dollar production mountain. Spend all this and there might not be enough money left over to keep Stealth development going at a sensible rate.

The only sure way out of robbing Stealth to pay for the B1 is for Reagan to suffer Carter saying "I told you so" and forget about building the B1. This would be betting all the chips on Stealth as the bomber of the future.

But suppose Stealth, like the Trident submarine and otehr super-weapons, were delivered years after the contractor promised it? Reagan could serve two terms and leave office in 1988 with no new bomber in the flight-test stage, much less deployed. Like Carter, Reagan would have to tell the bomber-hungry Congress: "Be patient. We're working on it."

It could be 1995 before any Stealth bomber, now called ATB in polite circles, for advanced technology bomber, is ready for combat.

This leads to option No. 3, committing the administration, and, it is to be hoped, Congress, to building the B1 for tomorrow and Stealth for the day after under two companion, fully funded programs. But even longtime Pentagon allies laugh off this prospect.

"He [Weinberger] doesn't have a prayer in hell of getting the money to build two bombers," said one senior member of the Senate Armed Services Committee. "We have too much stuff coming along to pay for that, and he knows it."

One military committee chairman, who did not wish to be identified, said that only if the Air Force is willing to give up such prized projects as the CX transport for taking gear to such hotspots as the Persian Gulf region and some cruise missiles would there be enough money available to finance the two-bomber approach.

Chairman Joseph P. Addabbo (D-N.Y.) of the House Appropriation subcommittee on defense said Congress probably will go along with two bombers this year but then sober up to the costs, forcing economies on the Air Force that would leave the B1 going ahead full blast but Stealth stretched out in a back-burner development effort.

This would be disastrous, in Addabbo's view, because he sees the country being saddled with an obsolete bomber after spending billions on it.

"There's not even a B1 factory anymore," Addabbo scoffed. "Rockwell," which built the B1 test bombers, "sold it."

Proceeding with the B1 and putting Stealth on hold until the political weather clears may sound like the sensible, middle-ground approach. But the realities of modern defense contracting put a different light on this.

Defense contractors organize special terms of engineers and scientists and managers to compete for the big Pentagon contracts. This requires pulling specialists off other, surer moneymaking projects, and costs thousands of dollars in speculative salaries and overhead. Defense executives are reluctant to keep such teams intact for contracts that fade into a long shot for their firms.

Although Rockwell, since it built the test B1s, has the inside trace to win any contract for building an updated version of the bomber, expensive teams have been formed by four different contractors to vie for the advanced technology bomber. Rockwell and Lockheed, builder of the SR71 spy plane, have agreed to team up to build a Stealth. Boeing and Northrop have joined in a rival team.

Weinberger is within days of making his recommendation to the president. "It's a tough decision," said Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee. That's Texas understatement.