Qualcomm Snapdragon 821 Advantages
Larger memory bandwidth (29.8GB/s vs 17GB/s)
Qualcomm Snapdragon 768G Advantages
Better graphics card performance FLOPS (0.576 TFLOPS vs 0.3343 TFLOPS )
Higher Frequency (2800MHz vs 2342MHz)
More modern manufacturing process (7nm vs 14nm)
Lower TDP (5W vs 11W)
Released 3 years and 9 months late