Qualcomm Snapdragon 821 Advantages
Larger memory bandwidth (29.8GB/s vs 25.6GB/s)
Qualcomm Snapdragon 778G Advantages
Higher Frequency (2400MHz vs 2342MHz)
More modern manufacturing process (6nm vs 14nm)
Lower TDP (5W vs 11W)
Released 4 years and 9 months late