Here's the 99c answer:

Sup32 is a performance-optimized and price-optimized version of Sup720. Sup32 is 32-Gbps bus-based architecture, whereas the Sup720 is a 720-Gbps switch-fabric-based architecture.

Both Sup32 and Sup720 are equipped standard with the PFC3B. This enables feature consistency and enhances operational manageability from the access to the aggregation and core edge of the service provider networks.

The key difference is the fabric versus bus enabled architecture. Fabric based has more high-availability features.

There area also fabric enabled line cards 67xx series that make use of switch fabrics architecture vs bus.