a router is needed to talk between IP subnets (usually only in a commercial situation with multiple subnets OR between your LAN and your ISP's LAN). If you're just running a LAN with one subnet you don't need to route.
Other than that, there is no performance difference.
BTW, a typical Cable/DSL "Router" is really a router (the WAN connection that goes to the cable or DSL modem) and a switch (the 4 or 8 port LAN side) together.
<i>It's always the one thing you never suspected.</i>