Speaking of Repair Permissions, are there any differences for that feature between Onyx and TechTool Pro?
Looking at the release notes for Onyx v3.15 (specifically released for El Capitan), I would be re-assured that the utility should perform properly for the functions it is designed to perform on an El Capitan install.
The Onyx developers have a very credible history of maintaining parity with OS changes as they are released.....the major caveat for most folks would be to ensure that the version of Onyx which they have is appropriate for the version of OS upon which they wish to use it.
As to how it compares under the hood to TechTool Pro v8.x, I couldn't begin to guess....maybe a side by side comparison of each running a permissions repair while monitoring the process using the Terminal and the Unix "top" command line tool????
Using the Terminal to monitor the underlying process, I ran both Onyx3.1.5 and TechTool8.0.3 permissions repair functions in order and saw that both make calls to the same Unix repair_packages tool....so apparently they seem to be identical in their approach.
(edited to add disk permissions repair comparison...)
