Contrary to Afghanistan, the communists had a significant popular support in revolutionary Iran. Tudeh was an important fraction of the revolutionaries; only that Khomeini managed to consolidate power before them, and then turned on his former allies.
With Khomeini dead and the Islamic part of the revolutionaries in disarray, the more probable situation is a mix of Algerian and Czechoslovakian one: Tudeh wins, spends some time in power consolidating, then organizes an election, loses it, and gets back to power nevertheless in a coup (financed and logistically supported by the Soviets).
If they don't manage to stay in the saddle after that and call Soviet Army, then we have an Iranian Civil War on our hands and the outcome described by Catmo is not improbable. But my stomach feeling is that with the large poor and lower-middle-class urban population, the Communists should be able to get enough power base even without direct Soviet intervention.
Afghanistan, by contrast, is mostly a village country with only Kabul deserving the name of a real big city - and most occupying powers or foreign-supported regimes in Afghanistan did not have a lot of trouble holding Kabul.