It really depends on how large your network is and what you need the devices to do. The router by default isn't as secure as the ASA is by default. But the router does have the capability to configure zone-based firewall which is stateful and is quite good if you ask me.
For efficiency I would say use both. Yes either one can perform your needed requirements (routing, vpn, firewall). Router is ofcourse better for routing, while ASA is better as a firewall. but for the most part I would look at your network size and work out from that. If you have a large network definately split the services and have the router do routing and the ASA vpn and packet inspections. if you have a small network, then there isn't any need for both unless you have plans to expand in the not too distant future.
Marius
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