They are essentailly the same, though you can't put a pix image on an ASA or vice versa. PIX is mages begin "pix....", ASA images.. well, you can guess.
There are some differences because of hardware - an ASA does not have a serial port for failover (so you use LAN-based failover), it doesn't have FO/R/UR licences, and interface IDs are different.
But in terms of NAT, ACLs, routes, object groups etc it's the same. You can port the config but watch interface and failover config.
