Definitely ASA is better choice because PIX is going to reach End of sale status...
ASA offers also more interfaces for same price (fe. ASA 5510 has 4 interface by default PIX 3)
In total, there are five models of the Cisco ASA
http://cisco.com/en/US/products/ps6120/prod_models_comparison.html
ASA also offers additional modules like AIP-SSM for Intrusion prevntion
http://www.cisco.com/en/US/products/ps6825/index.html
and SC-SSM for content filtering
http://www.cisco.com/en/US/products/ps6823/index.html
M.
hope that helps rate if it does