As everyone said, HL2 runs much smoother in general.
Not sure if it's accurate to say that it's because D3's graphics system is superior. They are different game engines entirely and both the product of the latest technology.
I understand that a great deal of effort went into making HL2 more scalable meaning that it would play smoothly a wider array of systems, be they newer and older systems.
The object being that more people would be able to enjoy the game and more people would be interesed in buying it.