If your ISP allowed it you would not need a router with this option. Personnaly, I don't like this layout. In B you have to make sure your internal router is configured to force IP traffic to the firewall. If anyone compromises your routers you are sunk. Routers have simple passwords and in my opinion are slightly more vulnerable, unless you disable inline administration - a pain.
Some organizations require authentication for outbound traffic.
Most firewalls do a good job of keeping up with a T1 line. Many will handle a large fraction of an ethernet link. Indeed, smaller routers are limited in their packet throughput. If you have many small packets they will be the bottleneck.