After six decades of central planning, communism has given the Soviet people a worse than stagnant economy and chronic shortages of consumer products, including even such basics as food, clothing, and housing.

Consequently, if there is any one point on which the entire Soviet Union agrees, it is on the need to scrap this bankrupt system and start embracing free enterprise.But just how far and how fast should the Soviets go in making this switch? On this point, there is sharp disagreement among economic experts and political leaders alike.

As a result, the Soviet Union finds itself at another crossroad, facing a crucial decision this weekend that could determine its economic future for many decades to come.

That's when the Supreme Soviet national legislature is to start considering the watered-down new economic plan that President Mikhail Gorbachev outlined this week. Only the latest in four such complex blueprints, the new one backs away from a 500-day timetable for switching to capitalism but aims instead at achieving the transition in two years without promising that such a timetable would be met. And it puts Gorbachev on a collision course with Russia, the largest and most populous state in the Soviet Union.

The new plan is being justified on the grounds that the faster 500-day schedule would bring on unemployment and widespread suffering, perhaps touching off violence. But the new plan promises less pain only by promising less reform. It would sell off less government property, do so more slowly, and have the government continue to foot the bill for industries operating in the red despite the already enormous deficit in the Soviet budget. Wholesale prices would go up, but retail prices would not rise until later.

More positively, it would give the 15 Soviet republics sweeping new powers to run the economy, free many prices from government regulation, and allow private ownership of businesses.

Dropped from the new plan, however, is a previous clear commitment to private ownership of land.

Seeing these changes not as a reflection of economic realism but as an indication of procrastination and of the bureaucracy's reluctance to relax its grip on power, Russian President Boris Yeltsin is talking about spurning Gorbachev's new plan and proceeding with the 500-day timetable just in Russia.

Despite Russia's size and importance in the Soviet Union, it wouldn't be easy for Russia to go it alone - particularly if Yeltsin is serious when he mentions the possibility of the republic's setting up its own currency, customs service and army.

If the Soviet Union were not so authoritarian and rigid, it might usefully consider the possibility of treating Yeltsin's plan not as a threat to the Kremlin's mastery but as an exercise in simply determining what works best. If Russia can switch to capitalism in 500 days, fine. But if the transformation really takes more time, so be it.

The trouble is that the Gorbachev plan and the Yeltsin plan for reforming the Soviet economy share the same flaw. Both would be imposed from the top of the hierarchy down. Long experience elsewhere suggests that fundamental changes are most effective and lasting when made from the grass roots upward.