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