Hi,
For situations where security is primary concerns, I would go with ASA than Router. That way the configurations (allowed/blocked ACLs) will be minimal on ASA when compared to routers.
Also, (Iam not sure if this changed in new IOS with routers), the phase1 lifetime is only 24hrs, but in ASA this can be changed to longer duration. You can even add SSM etc. modules in ASA.
hth
MS
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I read this somewhere..
"Cisco IOS firewall is often known as "poor mans firewall". If you want a filtering solution, use Cisco IOS. if you want a real firewall that does deep packet inspection, etc...use the Cisco ASA"