The question is more what are your requirements !
1.) IOS Firewall can support all kind of interfaces, serial, ethernet, ADSL ... PIX just has Ethernet and Tokenring.
2.) IOS Firewall has a lot of Routing features and routing protocols that PIX has not.
3.) PIX has more performance and is optimised as a Firewall appliance. IOS Firewall has or had a better application protection but this now equal with PIX OS 7.0.
See the definition in the SAFE Blueprint that talks about this features.
http://www.cisco.com/go/safe
Definition of SAFE SMB:
At many points in the network design process, you need to choose between using integrated functionality in a network device versus using a specialized functional appliance. The integrated functionality is often attractive because you can implement it on existing equipment, or because the features can interoperate with the rest of the device to provide a better functional solution. Appliances are often used when the depth of functionality required is very advanced or when performance needs require using specialized hardware. Make your decisions based on the capacity and functionality of the appliance versus the integration advantage of the device. For example, sometimes you can choose an integrated higher-capacity Cisco IOS® router with IOS firewall software as opposed to a smaller IOS router with a separate firewall. Throughout this architecture, both types of systems are used. When the design requirements did not dictate a specific choice, the design opted to go with integrated functionality in order to reduce the overall cost of the solution.
sincerely
Patrick
