Good question. Firstly you are correct to consider using Ancestry if you do go down the DNA route. That's because it has by far the largest DNA database of all the DNA companies. After testing you can widen your net by uploading your Ancestry test results to a few other sites. Ancestry, however, will not let you upload test results if you test with another company, so if you don't start with Ancestry you cannot access their database.
Testing should come with a warning however. You can get results that you don't expect and which can be very upsetting (I have personal experience). Nevertheless I have widened my family tree enormously as a result of testing, with cousins from families I had never heard of before.