In lighting, UL is a more common mark than ETL. But there really is little difference between the two, especially for those trying to pass inspection. Inspectors and Authorities Having Jurisdiction (AHJs) accept both, as both show compliance with the minimum safety standards put forth by NRTL.
It is, however, a common misconception that UL is the only acceptable mark of compliance.
Both UL and ETL are recognized as NRTLs, or Nationally Recognized Testing Laboratories. The U.S. Occupational Safety and Health Administration (OSHA) regulates and recognizes NRTLs.
According to many manufacturers who seek these certifications, gaining ETL listing tends to be a more expedited process than UL listing, despite the fact that testing is identical. And, in recent years, as technology has caused a proliferation of product, more manufacturers seem to be gravitating toward ETL as its listing laboratory, due to its quicker certification process.
In short, these days, ETL is often considered the more expedient testing laboratory of the two.