Ive run both on several Z32s that are still running years and thousands of miles later with zero issues. You might consider reaching out to Bernie as he sells OEM and sometimes aftermarket cheaper than others. I have also purchased from GMB on rock auto and Ebay with zero issues on the same GMB parts.
Obviously OEM will get more favorable responses. Now Gates is the manufacturer of the OEM nissan timing belt itself.. that can't be said about the idler pulleys afaik.