Well, I would have to answer that one with an 'It depends' :)
Seriously, strictly speaking the concentrator series boxes are for remote access VPN, but they can do site-to-site VPN as well in circumstances where it makes sense. Personally, if I have carte blanche, I prefer doing my site to site VPN tunnels via a router-to-router mechanism, using the PIX for the traffic filtering and inspection and using the concentrator for all remote access users (especially those who have high speed broadband access). From an IPSec perspective, they (the router, PIX and concentrator) all essentially accomplish the same tasks, but are tuned for different environments and some offer specific features that are more appropriate for certain deployments.
I would suggest you speak with your rep and SE for what solution makes the most sense for you.
