I would say the same thing - what crieria do you want to compare on.
But to answer your question "what is diff b/w PIX & ASA?" the difference is the name:-
PIX = Packet Internet eXchange
ASA = Adaptive Security Appliance
The PIX does not do:-
Advanced Inspection and Prevention Security
Content Security and Control Security
ASA has better clear text & encryption thru-put than the PIX.
Max Thruput - vpn thruput - model
150mbps 100mbps 5505
300mbps 170mbps 5510
450mbps 225mbps 5520
650mbps 325mbps 5540
1.2Gbps 425mbps 5550
5Gbps 1Gbps 5580-20
10Gbps 1Gbps 5580-40
but on a vpn tunnel between 2 locations the tunnle will only be as fast as the slowest connection? My customer is currently running a 501 pix for their site to site and upgrade to 3mb up/down on both locations will they see a huge speed increase by upgrading to the 5505 ASA for their VPN termination?