If you ask me, I would say that depends on the size of the organization. If you are a small org with limited budget, your focus would be on optimizing resources and upgrade to the FW image would be an option. However, if you are a large organization and security is important, the PIX is the way to go. The PIX is custom built to protect your networks, and provides better security (hardened OS etc), throughput, managibility(many more command options available) and a number of other other featues that are not available in the router with FW image.
