I can give some of the pros & cons I have encountered. IOS vpns are fine if you have a small amount of users. I say this becuase when you bought you router if you did not plan for this then you will really burden your router. If you have the memory and the encryption modules and vpn accelerator board then you should be fine. Advantage; move in cost is low for a small amout of users. One box to maintain and configure.Has the ability to pass muntiple protocols. Cons;high move in cost for large amount of users. VPN tunnels can distract the router forom it primary task, routing. Difficult to set up failover or load balancing.
VPN concentrators are great if you have a large amount of users. They are dedicated boxes for this task. You can add encryption modules and you can set up a pair to be redundant or load balance.Advantages; dedicated appliance for terminating vpn tunnels. Better performance. Redunancy and load balancing. Cons; very high move in cost for small amount of users. Time required to learn and correctly deploy. will only pass ip traffic.
As with both solutions you need to determine the client software you will use and the protocols you need to use. If you have less than 50 users cost is an issue and only need to use ip only and don't need any encryption then use pptp on a router packed with memory and has a fast processor, 2651 or 3640 for example. If you have more than 50 and need encryption get a 3015, if you have more than 100 then get a 3030 with at least 1 sep card.
Hope this helps
