Dedicated routers offer same-or-better performance and capacity of a "layer 3 switch", and give you FULL layer 3 traffic control, not the half-assed firewalling, prioritization, etc. that "layer 3 switches" provide (at slow fucking speed).
"Layer 3 Switch" is a marketing term for "half-assed router", and nothing more. If they were as capable as routers they'd be called routers because they'd fucking be routers. It's just like "smart managed switch", which means you get a semi-functional web interface (and maybe a semi-functional serial interface), good enough to do basic filtering, authentication, and VLANing, but it's no where near as capable as a true managed switch.
Saying a layer 3 switch is better than a router is like saying a spork is better than a spoon, fork, and knife.