Source vs Babylon.js

Valve's official documentation website is great for newbies. It demonstrates the pros and cons of the engine (and since the website isn't maintained by Valve, but instead the community, the pros and cons are largely unbiased). There are also a number of pages dedicated to entities used within official Valve games and also community-made mods that were turned into full-fledged games by Valve. These pages explain the ins and outs of how most source programming works. There are also guides for Valve's tools which are both included in Source SDK and in any Valve-developed game.

Since it has precalculated lighting, this engine is great for low end PCs too.

You don't need to code your own door or ladder etc. You can pretty much use every entity used in Half-Life 2 yourself easily.

Of course you can program stuff into source but for level design only, source has a really easy input/output system for your level logic (e.g. doors, trigger when player walks into room...).

Source includes Source Filmmaker, a video capture and editing application.

Thanks to the increasing popularity of Babylon, it has a growing community of helpful developers. It's easy to find help on their forum.

Babylon provides a playground where you can explore examples and play with the code.

The official documentation offers a wide variety of well-written tutorials on topics from beginner to advanced.

Additionally, there are many tutorials written by the community available that you can find by doing a google search.

Babylon has great project health, with activity on Github daily for bug fixes and new features.