Magellan is awesome. With regard to the Soviets, most of the geomorphology of Venus was already worked out via the earlier Venera-15 radar maps, studied by both Russian and American scientists. The Russians discovered coronas, arachnids, tessera. They saw there were not plate boundaries, and they did the first real meteor-crater count and learned that the surface of Venus is young (100 million years).
By the early 1980s, Brown University and the Vernadsky Institute were working very closely, so in fact James Head and Alexander Bazilevsky were PI's on both the Venera-15 and the Magellan missions. At one point, to get around Reagan-era restrictions, the state of Rhode Island and the Soviet Union actually signed a bilateral treaty to permit some of these cooperative activities between Brown and Vernadsky!
All that said, Magellan images are 10x higher resolution, and have a higher beam angle and more dynamic range. It also covered most of the planet. It is certainly America's most important Venus probe.
