Perhaps the most significant difference between Linux and the BSDs is the development model. Linux development, at least for the kernel, is wide open. BSD development is more tightly controlled. BSD advocates would argue that this results in systems that are more stable than Linux. I think this is only partly true.
Stability is certainly more consistent from one version to the next for the BSDs.
With Linux there is more variation. Most Linux kernels are astoundingly stable, but the wide-open model does result in a real lemon now and then. In the end, it doesn't really matter too much. The stability of all the freenixes is one to two orders of magnitude above that of the leading proprietary x86 operating systems. There are notable exceptions.