personally i prefer a pix whenever possible.
to determine which one to be deployed, we should identify what sort of feature is required. e.g. any vpn? any routing involved? going to connect to the internet or internal use?
assuming vpn is one of the requiremnet and not much routing is involved, i will go with the pix. from my experience, i feel that pix handles vpn better in terms of reliability and flexibility.
even pure firewalling i guess pix would still be my choice as pix is specially designed for security; whereas router firewall feature set is just an add-on feature.
