Source vs HeroEngine

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.

Backend completely handled by the team at Hero.

No need for Perforce, Github or any other versioning software. It is built in. Check, Compare, Merge, revert, etc.

Developers of the engine and game developers are active on the Forums and Skype for private messaging.

Able to create many types of games and scales all the way from a single area up to a full-blown MMO.

Designed for the needs of a MMO. HSL is also relatively easy to learn as scripting languages go. It was geared towards beginners, not time hardened programmers, and as such, its easier for a novice to pick it up sometimes, than a c++ programmer. That, and it was intended to be "readable" more like English.

Skype channel and forums questions answered by developers of HE and users very quickly.

Though the export pipeline may be different and currently requires 3ds Max or Maya, the format is highly optimized to reduce high quality models down to KB's.