In what field? UCD has both a medical school and a vet school, both of which offer many opportunities for biomedical research. Davis is also very strong in bio-agricultural research -plant physiology, soil science, wine-making, etc. UCSB has some strong programs too, but in biomedical I'd give the nod to Davis.
^^In that case you are better off at Davis. UCSB has neither a medical school nor a veterinary school. Davis does. Back when I was there, we used to have undergrads doing research all the time in our medical school lab at UCD.