The Source Engine has highly modular characteristics that allow it to retain a high level of detail and a competing edge against newer graphics engines. However the brush-based world creation and created-model basis for things such as high-complexity objects, flora and characters limits the abilities of the game entirely to artist or user-created objects; this operating method constrains the scope of the engine from ever doing anything procedurally generated, and possibly as well dynamic loading.
The whole brush-based world creation is also starting to show it's age; the engine is six years old already. In all likelihood Half Life 2 Episode 3 will be the last game for Source before Valve retires it and makes a new one. Although their capability for innovation astounds me, it would both surprise me and not surprise me if a new engine they make is still for 32-bit DirectX 9. Which is to say they'd be doing more with less, seeing as Half Life 2 Episode 2 stood on it's own in a fair round against the CryEngine, who optimally operates on 64-Bit DirectX 10.
The differences?
Left is HL2E2. Using Baked Ambient occlusion attached to the level gives more realistic lighting without putting a dent in the render budget. The trees are more efficently designed albeit static. HDRI is implemented on all lighting solutions.
Right is Crysis. All vegetation is procedurally generated with jigglebones and leaves have subsurface scattering and dynamic volumetric lighting and dynamic occluding shadows as well as high-pass motion blur. Depth of field illusorys add realism to the gun.
The biggest difference is that HL2E2 is going to have a far smaller file size on your hard drive because of more efficient textured and modeling methods. Apart from that the verdict is up to you.