For reasons I discuss elsewhere, blood testing is now my recommended choice. Remember: unless you are under 21 or on DUI probation you are free to refuse the handheld Breathalyzer given at the scene of the DUI stop AND I advise that you so refuse.
The first problem with blood testing for alcohol is that it’s impossible to tell how much alcohol was present at the time of the blood draw and how much alcohol was created by later fermentation of the sugars in blood by microorganisms. In other words, the microorganisms naturally present in the blood begin to produce alcohol in the sample.
I used to say that it didn’t matter which test you took, blood or breath, because both of the tests were subject to attacks based on scientific problems with the tests themselves. As a result of the recent decision of the California Supreme Court in People v. Vangelder, I have changed my recommendation.