IOS images are lower in cpu and memory so you can fit more of them in one topology.
IOS images are usually run in Dynamips however R+Svms are housed in a virtual machine.
What are the differences? Here are the major differences: These are classified as Routing and Switching virtual machines (or R+Svms) for short. In the new GNS3 1.4, there is a way to run a second category of switches and routers. For older images, we use and maintain Dynamips an emulator dedicated to emulate some Cisco hardware. GNS3 offers multiple ways to emulate IOS.