I've had my YDNA tested twice. Once with ancestry.com (a 41 marker test) and again with Family Tree DNA (a 37 marker test). The test results were virtually the same (duh). The results were off slightly on two markers.
One test would have been enough but I went with ancestry.com first and then decided I wanted to be part of the Family Tree DNA Weeks Surname Group and to be part of the group you have to have your YDNA processed by FTD.
Here are my results: (click on image to enlarge)