Several companies do autosomal DNA testing for genealogical purposes including Family Tree DNA, Ancestry.com, 23andMe and the Genographic Project Geno 2.0. ISOGG has a very useful comparison chart put together by Tim Janzen: ISOGG Autosomal DNA Comparison Chart.
I have used Family Tree DNA for my family, a friend has tested with 23andMe and a relative in the USA has tested with Ancestry.com. So, I am most familiar with Family Tree DNA.
As a person living in Ireland some issues come to mind:
Ancestry.com does not send kits outside of the USA. 23andMe attracts a lot of people who test for medical reasons as this is built into their testing. Also, although the 23andMe Relative Finder kits are relatively less expensive than others, there is an incredibly large postage applied to mailing kits to Europe. Family Tree DNA kits are more expensive but postage is less. So, living in Ireland, I can’t get an Ancestry.com kit. A kit from 23andMe is $99+$80 (approx) giving an approx total of $179 (approx). Family Tree DNA’s Family Finder autosomal DNA test is$99+$6 postage (for outside the USA). The Geno 2.0 kit which is available worldwide is $199.95 plus a shipping charge of $20 (as per the chart on ISOGG). Note prices change over time and should be checked.
GEDmatch.com is a voluntarily run website where you can upload autosomal DNA results from Family Tree DNA, 23andMe and also Ancestry.com. This website is as vital to autosomal DNA testing as Ysearch is to yDNA testing. So, Gedmatch enables people from many of the various testing companies to compare results with others who have also uploaded their autosomal results to it.
Some websites and blogs which would be of benefit in the decision about which test and which company to use are:
It is a minefield for the unitiated but price and availability are two critical factors for those of us who are not in the USA!