What's your preference for importing, fbx or blend files? Or under which circumstances do you choose one over the other?

I rarely if ever use Blend files in Unity. FBX is what I use as I find it simpler. I believe with Blend files it imports everything but with FBX you can export out specific meshes out of Blender. It's been a while since I've used Blend files in Unity though so this may not be entirely accurate.