Technical differences between Snapdragon 820/821 and Snapdragon 835

Qualcomm had quite a bit of bad luck with Snapdragon 810: The processor was a hot head and was not a strong performer. This was thanks largely to the production process. The direct competitor of Samsung, the Exynos 7420, which was included in the Galaxy S6, was developed with higher manufacturing standards and had its advantages in terms of power consumption and heat production. The Snapdragon 808 was less powerful in comparison, but was better at controlling the heat it produced. 

Is the Snapdragon 821 a good choice considering that the Snapdragon 835 will be available in a couple of months? The technical differences between the two are obvious: Qualcomm uses a middle path between ARM's cores and a Qualcomm-based ARM-compatible architecture. The Snapdragon 820/821 also introduced Qualcomm's first custom 64-bit CPU core, Kryo, which are largely based on the ARM Cortex A72 or A73 with additional optimizations.

Source: ANDROIDPIT