Most people think a Health Care Proxy and Living Will are the same thing and you only need to create one. In New York, a Living Will is not a legal document, but it is still best to have these two documents drafted so that they can be interpreted in conjunction with each other. The best advice I can give is that you should create a comprehensive estate plan, which includes among other documents, a Health Care Proxy AND a Living Will.
Since I am admitted to practice law in New Jersey, Connecticut, and Illinois as well, I want to point out that advanced directives may have different names in different states. In New Jersey, a Health Care Proxy is known as a Proxy Directive and a Living Will is called an Instructive Directive. In Connecticut, you can have one document which does both, called a Consolidated Health Care Instructions and Advance Directives. In Illinois, a Health Care Proxy is known as a Health Care Power of Attorney and a Living Will is known as the same in New York and Illinois. One major difference is that in New York you can account for mental health treatment in a Living Will, but in Illinois, you would use a separate document, called a Mental Health Treatment Preference Declaration.
Going back to New York advanced directives, a the Health Care Proxy allows you to appoint an Agent to make health care decisions should you become unable to make them for yourself. A Living Will provides the information to your Agent, or any other health care provider if you do not have a Health Care Proxy or do not have anyone you feel you are comfortable appointment as an Agent.
A Health Care Proxy gives a lot of control to your Agent because it gives control over your health care decisions, generally. A Living Will provides specific instructions and requires you to know which health care decisions will arise. A Living Will can provide more information than just your feelings about CPR, artificial nutrition/hydration, and organ donation. My separate blog on just Living Wills will go into more detail on what you should put in your Living Will.