My hypothesis:
SOT of Snap625 + 4000mAh + 1080P is more or less similar to current Redmi 3S. This is due to the Snap625 is more energy efficient when it comes to multi-tasking and heavy intensive tasks like 4K@30fps HEVC video capture and playback, gaming and GPS Navi without badly sacrificing performance. There will be "micro-stutter" or "micro-lag" for hyper-intensive tasks but can be "forgiven" in the trade of EXTREMELY low energy consumption. Thanks to the 14nm fab for Snap625.

Don't forget the fact that the version with SD625 will also come with Full HD screen Instead of the usual HD for other version of Redmi 4 and Redmi 3. The screen alone might be more than enough to offset any energy savings from the use of SD625. But it will certainly be a lot better with any off-screen task.

About the same performance wise but 650 has better cores and gpu, bigger screen vs smaller. Depends. 625 has even longer battery life ,so... Long battery life and smaller screen. Go redmi 4 prime like i did.