Main defference is ASA has slot for service modules (AIP and CSC) more info:
http://www.cisco.com/en/US/products/ps6825/index.html
http://www.cisco.com/en/US/products/ps6823/index.html
So it offers more complex security services (including antivirus, antispam solutions..)...There is no annoucement about end of sale for PIX I think they can coexists for several years....PIX 525 and 535 has advantage with number of interfaces
ASA with modules is more expensive but offers more services.
great is that they share same operating system
M.