My wife did a lot of research into this when we picked an engine for our website and she found that the API of ThreeJS breaks backwards compatibility a lot, while Babylon is exactly the opposite. Babylon also has vastly superior documentation and doesn't seem to suffer from lag spikes as much. However, it's an oldschool API (did I mention they really value backwards compatibility?) and requires quite a bit of learning to get going. Also, it's three times the size of ThreeJS, so bandwidth optimization is absolutely required.