I own a D3s and a D300. I used the d300 purely as a backup. My thought, sell the D300 and purchase the D800 (which i have on order). If i add the vertical grip, I can get almost as many fps as my D300, allowing it to serve as a backup to my D3s.
Now I can use the D800 as my primary Portrait camera, with D3s as back up, and D3s as Sports camera, with D800 as backup. I already "detune" the D3s to 8 fps for all but the fastest sports just becuase any faster and I seem to just collect a lot of very similiar shots. Then i get to PP all those shots. Another option, wait for the D400, which i have been waiting for for a couple years no , I think i will quit waiting and jump in.