Source vs S2 engine HD

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.

It is possible to edit weather conditions (rain, thunders,wind, clouds, night and day cycle) by simply moving some sliders.

Important functions can easily be seen and accessed, making the engine quite user friendly, even for beginners or those who are new to the engine.

S2ENGINE HD rendering power is comparable to more famous engines like Cryengine and UE4.

S2ENGINE HD is integrated with a complete terrain editor that lets you sculpt, paint, add vegetation, and add splines like roads, rivers, fences, etc.

Essential functions are very easy to use and understand.