How do the Surround plug-ins differ from the Stereo plug-ins?

Surround plug-ins offer more channels than Stereo. The surround algorithms have much more extensive control of early reflections. There are multiple patterns and many more ways of distributing those reflections. This is especially useful for mixers doing ADR or foley work. Finally, there are additional delays and gain controls to allow these reverbs to work in complex multi-channel mixes.