If I remember correctly, they used animatronics (machines & CG) for Jurassic Park, and went all-out CG in The Lost World. To begin with, the texture of a real-life object will ALWAYS look more realistic than a CG texture IMHO because it has infinite detail. Look at the skin on the back of your hand for a second and see how much detail is in that. Go outside and look at the grass in a field, or the branch of a pine tree. INFINITE detail, right down to a molecular level. Detail as far as the human eye can see.

Secondly, a major criticism I point at CG characters and creatures in movies is that they move too fluidly to be real creatures with real skeletons. In many ways, living creatures are like machines in that we are skeletal frames covered with skin. Our skin stretches and moves around our skeleton, but it is not completely fluid. Our movement, which is made possible by our brain and then our muscles, is limited to our skeletons. Humans and animals do not move with complete fluidity, they move like pieces of machinery moving in perfect unison. That is to say that there is a machine-like 'jerky' quality to it. THAT is why I believe animatronics looks more realistic than CG alone.