The difference is mainly speed:
The Cisco 7600 series router is an edge aggregation router, and the SIPs provide a cost-effective solution for customers seeking moderate- to high-port density and line rate services:
•The Cisco 7600 SIP-200 provides WAN edge aggregation through lower-speed and low-density SPAs for network environments requiring regional office connectivity to headquarters, or collapsed LAN/WAN deployment.
•The Cisco 7600 SIP-400 provides higher-speed, high-density link aggregation for network environments requiring leased line and metro aggregation.
•The Cisco 7600 SIP-600 provides a high-speed interface for WANs and metro aggregation.