Unless you’re doing extreme pixel-peeking, or printing billboard sized images megapixel count is not really relevant anymore. Sensor size is important is a slightly larger distance between sensor elements results in a better image, but again, that’s only really for people delving into the extreme limits.
The bigger issue you’re going to have is the comfort of the subject and both camera proximity and light affect that.
Eyes saccade (quick small involuntary movements of the eyes) even when someone is trying to keep their gaze steady, so you need a relatively fast shutter speed. This means either lots of light, which is uncomfortable for the subject, or a wide open lens, which means a more shallow depths of field.
The 200mm lens lets you keep a greater distance with the same coverage area than the 105mm lens. This allows for both more comfort on the part of your subject as well as a slightly deeper depths of field for the same f-stop.
Regarding the camera, no real difference, both take excellent quality photos.
Personally, I’d go with the 200mm lens and offset lighting with the lens open pretty wide to allow for a fast shutter speed.
Realistically, you’re going to have to experiment to see what works best for both you and your subject.