Xeon E7 2820 's Advantages
Higher specification of memory (1333 vs 4)
Larger L3 cache size (18MB vs 1.5MB)
Intel Xeon D 1521 's Advantages
Released 4 years and 7 months late
Higher base frequency (2.4GHz vs 2GHz)
More modern manufacturing process (14nm vs 32nm)
Lower TDP (45W vs 105W)