The differences is basically processing power, disks and memory. However, the actual optimization would be roughly the same for similar traffic. The 612 uses the same software and compression technology as the 512.
So in WAAS 4.1.x, with all the AO enabled: the 512 (depending on memory options) can handle maximum optimized tcp connections from 600 - 1200. The 612 (again, depending on memory) supports 1600-4800 connections.
Obviously the 612 has more horsepower and can be used in larger offices and for larger WAN links.
Hope that helps,
Dan