The answer is both: Better together!
Animal testing is not an alternative to human trials, it complements it. Medieval castles had high walls and soldiers in them – both protect the defenseless people in the keep. Sometimes high walls and soldiers were not sufficient, and the castle was sacked, but no one would conclude that high walls were pointless and that everyone would be safer if there were just the soldiers. In reality, doing away with the castle would mean more soldiers would die, just as doing away with animal tests would likely lead to more deaths in Phase I and II clinical trials; the consequence of this would be that fewer people would volunteer for clinical trials (just as fewer soldiers would wish to defend a low-walled castle).
We use a variety of methods in biomedical science – computer simulations, tissue studies, animal models, clinical trials, epidemiology etc. Different methods can teach us different things and the results are often used in combination to build our knowledge and understanding of physiology and disease. The same is true in safety testing – all methods of screening drugs have advantages and drawbacks, but if we use them effectively, in combination, we can see that safe and effective drugs make it to market.
Computer models are used to screen and determine the toxic level of a substance in the beginning of an experiment and tissue and cell cultures have become valuable additions to the array of research tools and techniques. However, animal testing remains a necessity. For example, blindness cannot be studied in bacteria and it is not possible to study the effects of high blood pressure in tissue cultures.
The same methods that have been developed to prevent and treat diseases in human have improved the lives of countless animals. More than 80 medicines and vaccines developed for humans are now used to treat animals. Animal research has helped develop many animal vaccines to fight diseases such as rabies and distemper in dogs and cats, feline leukemia, infectious hepatitis virus, tetanus, and has assisted in the development of treatments for heartworm. In addition, animal research has helped preserve nearly extinct species such as the California condor and the tamarins of Brazil due to new reproductive techniques being applied to endangered species. Translational studies – dog and cat models recapitulate what is seen in mouse models and vice versa.