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. Configure GNS3 to use an additional remote server.