Qualcomm Snapdragon 425 Advantages
Lower TDP (5W vs 11W)
Qualcomm Snapdragon 820 Advantages
Better graphics card performance FLOPS (0.3194 TFLOPS vs 0.024 TFLOPS )
Larger memory bandwidth (29.8GB/s vs 5.34GB/s)
Higher Frequency (2150MHz vs 1400MHz)
More modern manufacturing process (14nm vs 28nm)