It's a shame, as mods are a huge part of why Valve is what they are today. it simply feels as if they do not care about their free tools anymore. The other tools can have the same said of them, OBSDK's Hammer (the level editing tool) now has multiple render bugs that did not exist in previous SDK versions, etc. They have literally gotten worse as time has passed for example the demo smoother tool (used to edit the camera movement in a demo to get good footage for movies such as trailers), has actually become harder to use from the Episode 1 SDK to the Orange Box SDK. The tools are simply far too outdated and unkempt at this point. Personally, speaking as someone who has worked on multiple Source SDK mods since 2005, I would stay as far away from source as possible. Valve has not done a great job (I am putting this lightly) at maintaining the SDK lately and the tools remain in a near-broken state. If your game is not going to be commercial, you can use the Source engine for free, but then you will have limited access to the engine and be at the mercy of the SDK tools. From the jist of your post I assume you aren't going to be commercial, though. Well, by far and away your biggest question should be: is my game going to be commercial? If so, then you will have to obtain a Source engine license, which is going to be (in most cases) substantially more costly than other engines, especially options like Unity.

Source engine sdk