PoS is far more advanced approach since it demands much less electricity. However, this approach is good when it's applied from the first days of project's life. Live migration from PoW to PoS is very painful since it demotivates miners and inevitably leads the project's to its fatal split.
Depending on who you ask. POS is may be a step backward. A lot of stalled projects are POS based which looked good when they started.