Maybe OCCT's gotten better in the past few months/years, but I've heard that FreeCAD's been kind of rocky with bugs because of it. It's also decades old, which is a double edged sword from my point of view; while feature rich, it may be difficult to add new features, understand old ones or implement Quality Of Life (QOL) enhancements, as seen in FreeCAD. I've also heard that OCCT also takes a considerable amount of time to learn.
Fornjot is the new kernel on the block, has similar goals to this project and seems to get a considerable amount of progress done each week. It could also help both projects progress, as explained in this blog post.
I've still got a lot of work to do before I even need to start interfacing with a geometric kernel, so there is the option of delaying the decision until I get to that point.