From the Mayo Clinic website about CVS:
"Most test results are normal, which can ease anxiety about your baby's health. Occasionally, test results are unclear and amniocentesis is needed to clarify the diagnosis. With chorionic villus sampling, there's a rare chance of a false-positive test — when the test is positive, but no disease exists. It's also important to remember that chorionic villus sampling can't identify all birth defects, including spina bifida and other neural tube defects."
Which means, I believe, is that a decision to terminate would not typically be based on positive CVS results alone, but would require an amniocentesis to be sure the results were not a false positive. You would also not know if your fetus had spina bifida or other neural tube defects.
The Mayo Clinic says this about amniocentesis:
"For genetic amniocentesis, test results can reliably rule out or diagnose various genetic disorders, such as Down syndrome. Follow-up testing isn't usually necessary."
Because of these differences, and the significantly lower chance of miscarriage from the procedure, I chose to only do the amniocentesis. I was 40. It was quick and painless and gave me clear (positive) answers. The only drawback with the amniocentesis is that the CVS can be done earlier. But I wanted clear results, not several weeks of fear and uncertainty between tests because of a false positive.