Choosing a Y-DNA Testing Company
I'm going to let you in on a little secret. You need to consider what your goals are when selecting a Y-DNA testing company. If your goal is to find matches with others having your same surname, then you need to find out which company that project is using.
The Thornton Y-DNA project uses FamilyTreeDNA. This decision was made years ago. Since that time, many other companies have come onto the scene. It's tempting to test with one of those when the price looks better. The problem is that by testing with another company, you have missed out on joining the surname DNA project and having your results in a common database for comparison. There are some options out there like Y-Search which allow persons who have tested with other companies to compare results and upload GEDCOMS, but it still does not make one a member of a surname project. FamilyTreeDNA will not put the results from other testing companies in the surname project databases for quality assurance reasons. (They, of course, have provided Y-Search as a comparison tool, but they still encourage persons who tested with other companies to have tests redone at a discounted rate. The link is available at Y-Search.) I would be remiss if I did not point out that FamilyTreeDNA does offer a Genographic Project conversion option which allows one to join a surname project in the process of transferring data.
Most of the DNA testing companies run specials from time to time. Be on the lookout for one of these great deals. Many of the project administrators will post this information to their project web sites (so keep checking those sites). Some will also post to messages boards or mailing lists if they are allowed to do so. (It's probably a little difficult to promote a FamilyTreeDNA test on a message board or mailing list hosted at competitor Ancestry, so you might want to check GenForum for those notifications.)
If you plan to join a surname project, get your Y-DNA testing done by the company they have selected. Don't make the mistake others have made by going with what outwardly appears to be a good deal but might not provide the desired outcome of your testing.