The Snapdragon 660 is used in several high-end phones while the 630 isn't getting much love. Both are mid-range (so affordable) 14nm chips (so power-efficient), but only one of them uses the faster Kryo cores. A new chip may be coming to bridge the gap between these two. The Snapdragon 635 will also be built on a 14nm LPP process and will use an octa-core Kryo processor - 4 big cores, 4 small. The rumor does not mention clockspeeds, though (the two clusters of the S660 run at up to 2.2GHz / 1.8GHz). The GPU promises to be 20% faster than the Snapdragon 625 GPU. If true, that's not much...
Source: http://www.gsmarena.com/rumored_snapdragon_635_chipset-news-27271.php
