First a caveat - my primary desktop genealogy program is GRAMPS (gramps-project.org), which I have used exclusively for over a decade. Occasionally I do use GenealogyJ to load and check GEDCOMs that I receive from others, such as people with DNA matches. The question that I was answering addressed tools specifically for examining and/or working on GEDCOMs and since both GenealogyJ and Ancestris use a Gedcom file as its database they are useful for that purpose. While I installed and did some testing with Ancestris I haven't really used it in any meaningful way but it seems to work similarly to GenealogyJ and may be better supported. While both programs are useful for viewing and/or editing a GEDCOM I personally wouldn’t use either one as my primary genealogy program unless it is for a smaller database, i.e. hundreds rather than of thousands of people. Also, while I do use GenealogyJ, I find it a bit arcane, which may be mostly due to my not using it regularly.
I hope that this answered your question.