My friend Louis Kessler has published a comprehensive comparison of his results from 5 different DNA testing companies. I don't pretend to understand everything he says about the five tests, but I can appreciate the time and effort that has gone into the comparison. If you want to know exactly how the five companies compare in great detail, this is the first very comprehensive analysis I have seen. At the end of the post, Louis points out that he is getting a whole genome sequence test done and will compare the results from that test with all five of the other tests.
If you are really interested and knowledgeable about the details of DNA testing, you need to read and study this great post.