Hi,
Hm.......tricky question :). PIX 515E has a full firewall feature, can be add to 6 interface. 1721 router, yes it's router so if you plan to used as a firewall also good enough, but I think it's not having all the firewall feature, has a serial connection off course. When to chosee PIX and when to chosee IOS FW. Buy a firewall when you want firewall then buy a router when you want router.
A good firewall is not just a firewall it;s self. But more to the human concern. How the policy is implement and tested and how the policy will be maintenance.
HATO
