Both are private organizations to my knowledge, but the CDC (Center for Disease Control), which is a component of the US Department of Health and Human Services, endorses the IDSA
guidelines.
In other words, the IDSA point
of view is the mainstream,
and what you are most likely to encounter at your doctor's office.
Basically, there is so much squabbling over the definitions, testing, and treatments for Lyme disease that many patients (self included) fall through the cracks.