Although I have not had my DNA yet, I plan to have it tested with African Ancestry -- as I believe they are the ONLY company connecting African descendents to groups in present day Africa. The company and test to take depends on the type of info you are looking for.As you said, test are not cheap so I suggest before you send DNA samples, contact the company to see exactly what info they can provide. Tell them what you are looking for and they will tell you if they provide it. My opinion for instance, if you believe you are a descendent of the say 'Jefferson' family and that family has a DNA Project established, check the company that provides 'halogroups', ie L16; H5, and a long string of letters and numbers. If you wish to know the migration route of your ancesters (begin on the River Nile 100,000 years ago, travel through Persia-75,000 YAG; went to Greece; crossed the Atlantic 1619), check the with genographics company. If you believe your ancestors were from various ethnic groups, check the companies that would tell you -- ex.45% African; 10% Asian 5%; Native American or whatever else. (Do not know if they have DNA for Natives Americans by Ethnicity). If you want to know where your roots are from in Africa, it's African Ancestry.com. As far as I can see, the other companies' info is confusing and requires a lot more research. African Ancestry will tell if your DNA is Kru, Mandinka, Fulani, Amhara, Wolof, etc, Nigeria, Cameroon, Angola, etc. or non African. AA is expensive, 300+, but I believe I will get what I am looking for. When I'm ready to know my kinship to the Surname Slaver, I will join that project (if available) to see if my family match theirs.
Anyway this is my take on genealogy DNA research.
In Search of Roots -- Peace moy