The Cisco Pix firewall seems to be an actual hardware firewall built specifically for this purpose, while a linux firewall is an actual computer that acts as a gateway/firewall to the internet in a similar fashion. Both would seem to provide very good protection if set up correctly. The main advantage of the linux firewall would have to be cost. For more info on the Cisco Pix go to
http://www.cisco.com/warp/public/cc/pd/fw/sqfw500/