It used to be that the PIX would not allow a connection remote to remote and the concentrator would. In the 7.0 release Cisco addressed this. So it is not much of a factor now.
So I would say that one of the differences is that the PIX is a box that is optimized to do firewalls and also does VPN while the concentrator is a box that is optimized to do VPN (and that is all that it does).
It is my understanding that the concentrator is capable of handling more concurrent VPN sessions that the PIX is. (But how many organizations put that many VPN sessions on the box?)
The interface for configuring and managing the devices are very different between the PIX and the concentrator. (But which one is more preferable is a matter of personal preference)
HTH
Rick
HTH
Rick