it depends on needed features and on the number of BGP routes received from the upstream providers
IF you need to perform NAT you have to use a router as NAT is only supported on C6500 with Sup720 or better.
If the number of BGP routes is low ( < 5000) you can think of using a multilayer switch.
Again a full internet table of 400,000 IP prefixes would require a powerful router or a C6500 with Sup7203BXL or better
Generally speaking, the multilayer switch is much superior in the forwarding plane then a router but it is weaker in the control plane (because it works in hardware it needs to be able to store CEF info for all IP prefixes and the CEF tables are limited in size) and in the number of features supported (NAT is not supported).