Xeon E7520 's Advantages
Higher specification of memory (1333 vs 3)
Larger L3 cache size (18MB vs 12MB)
Intel Xeon E5649 's Advantages
Released 11 months late
Higher base frequency (2.533GHz vs 1.866GHz)
More modern manufacturing process (32nm vs 45nm)
Lower TDP (80W vs 95W)