The auDNA test from MyHeritage may be the best value right now, especially with their $49 Black Friday sale ($49 Amazon). In addition to the MyHeritage reports, you can also upload the results to FTDNA and get all of their reports for free.
What do I get if I transfer my autosomal DNA from MyHeritage™?
After transferring your results, for free, you will receive a list of your autosomal matches from our database and have access to our Family Finder - Matrix. The Matrix feature allows you to select and compare the autosomal DNA relationship between up to ten of your matches at one time. In addition, you will also be given full access to our Family Finder features, which include the Chromosome Browser, myOrigins, and ancientOrigins at no additional cost.
Normally this costs $19, but MyHeritage may be getting this special treatment because their tests are done by Gene By Gene, FTDNA's parent company.
If the size of the user base is your most important consideration, then Ancestry.com still has that market cornered. MyHeritage/FTDNA/Ancestry.com use the 'older' version of the testing chip. 23andMe/LivingDNA use the 'new' version of the chip, but there are currently FAR FEWER people who have tested with this.
Note: MyHeritage charges $12 for shipping unless two or more tests are ordered. No charge for shipping on Amazon Prime.
See current thread for Thanksgiving sale prices.
