I've worked with both the HL2 engines and the D3 engines. Trust me, the D3 engine is much better to work with. The last release was laggy mainly due to some un-optimizations, this will be significantly corrected in the future trust me. The HL2 engine's shadows are severely SEVERELY limiting. The lighting is based on the old burn into the texture type and the new dynamic shadows recently implemented are just as laggy in terms of performance as the Doom3 shadows. A comparison cannot be drawn between the two engines, but I assure you that this type of game would not be applicable to the HL2 engine at all.