How do I decide whether to have amniocentesis or cvs?

Personal decision. Your decision should be based on your age-related risk for chromosomal problems (higher the older you are), how important it is to you to get early information, and how much risk you are willing to take in finding out the information. Generally cvs will give you information one month before an amnio but has a risk of miscarriage triple that of an amnio.
If at increased risk. It is only indicated and may be considered in which there is an increased risk of genetic disorder. If you are 35 or older by the time your baby is due if you previously had a baby with down syndrome or another disoeder like spina bifida and you and your partner are known carriers of genetic disorder like cystic fibrosis.