In most states, you have a choice of breath or blood. Only a few states offer urinalysis. If you choose breath, many jurisdictions permit you to have a second test of blood or urine because a breath sample is not reserved after you blow into the machine and therefore cannot be re-analyzed by an independent lab at a later point. In California, only blood or breath is offered, unless neither is available in which case urinalysis is possible; a blood sample can be taken if requested after a breath test is given.
Analysis of a blood sample is potentially the most accurate. Blood samples also provide the greatest opportunity for your attorney to discredit the results. The vile used to hold the blood must have an appropriate amount of chemical in them to preserve the blood and prevent it from coagulating. Also, there needs to be an accurate and consistent amount of blood drawn. Plus, there are procedures that must be followed by the blood draw technician for the test to be reliably analyzed at a later date by the chemical lab.
Breath machines are susceptible to a number of problems rendering them often unreliable. The least accurate by far, however, is urinalysis. Thus, if you are confident that you are sober, a blood sample is a wise choice.
Urine is the least accurate and most easily impeached. This is the best option if available and if you believe your blood alcohol concentration is above the legal limit.