Have used D800 and D3s. The D3s is great in low light, high ISOs, high shutter speeds. The detail on the D800 is outstanding. Have used for nature photography and, with grip, find the speed just fine. Like the crop. The drawback is I only use to ISO 800 (my self imposed limit as the camera is more than capable at higher ISOs) as noise is more of a concern with the D800 than the D3s for me. For low light and higher shutters -- I usually use the D3s.
Can you rent or borrow both bodies to try before purchase?