In regards to Source 2 - Look into The DOTA 2 Alpha Mod tools set available on steam in the tools section.
Hammer has been completely remade and I am guessing the same will be done with faceposer. Valve this time around are doing everything themselves which is really awesome. Not to mention that they will be using VULKAN aka OpenGL next and the best bits of AMD's Mantle.
As for Unreal 4 - It is flat out awesome. I am not any form of tech artist but Unreal 4 lets me stroke my ego and lets me think so.
Here is what I mean -
Everything is visual down to the Shader Editors, Blueprint Visual Scripting, Behavior Trees, Animation Editors, Cinematic Editors, etc. It really empowers artists to do their best stuff which is to just make awesome art without having to rely on a programmer.
That said - If you are an Programmer you will be right at home with UE4 due to its C++ base which is highly extendable. Best of all Epic Games is not content. I don't say this much and I have used allot of game engines from allot of companies - I LOVE Unreal Engine and I want to marry it if I could do that. Just due to the amount of time that it saves me.
They also have a lighting / shading system that you made have heard about called PBR (Physically Based Rendering). So what does this mean?
It means that you spend more time working on making awesome art opposed to worrying about how it looks.
Here is the long version of what I am trying to say.
Anyway I gave my 2 cents - I would love to see Apocalypse Rising, Reborn, and War of Assassins come to Unreal 4.
I would be willing to lead such a charge.
EDIT: It is also worth mentioning that Unreal Engine 4 can do some crazy large worlds.
The demo that I am referencing.
Twitch.TV broadcast where they state how large the world is and What they did to make it. Skip to 45 minutes in that is when the talk begins.