I worked on VxWorks and Linux on Intel's IXP embedded processors before, and VxWorks' performance and memory footprint were always better than Linux's. For 'Performance' I refer to network throughput because IXP was meant to be for routers and gateways. It was around 2006 so I am not sure what these two OSes are up to now. Don't flame me because I am just speaking from my own experience.