HL2's coding side is supposed to be very similar to HL1's. That was valve's intent, to make it's modding tools and modibility the same to that of HL1, but they fail to realise that HL1's modding tools are arse. They should have IMPROVED those legacy tools for HL2 instead of regugitating them. Just because HL1 had the biggest modding community doesn't mean it's tools were the best. Doom 3's tools are incredable, yet doom 3's modding community is dying very quickly.
The primary questions you should ask when deciding to switch engines are:
"How large is the community of the engine/game in question" (the most important aspect. If the community is dying, you'll just be wasting your time) and "Will the fans appreciate this?" . All other questions like "How good are the tools?" or "What kind of graphical effects does it have over the current engine?", are secondary.