What is the second one?
My impression is they share patches enough, that they can't really be counted as different browser engines.
But perhaps that has changed?
The question was: What are the differences?
- Memory usage
- Energy usage
Then open https://browserbench.org and do the benchmarks.
Webkit will dominate Blink (and for that matter Gecko) in all categories.
My answer was "they are different" and implicitly "yes", partially because of the heavy development work that has been done since the fork seven years ago.
If you look at the featuresets and when they introduce features I think it's reasonable to conclude they have diverged enough that they can't just pull patches from each other.
Also, there was a webkit mailing thread back when they introduced their cut-down version of service-worker that said that most of the code from blink was basically unusable for them because of the differences.