Upload & Sell: Off
Depends on what your budget is...
D800 or D600 aren't realy "sports" bodies. The choices would be D700 (uses the same grip as the D300s), D3 and D3s.
A few other comments:
- D300s being a crop sensor gives you the extra 1.5x reach on your lenses. Going full frame on your spare body (assuming you do not keep the D300s) you will loose that.
- D700 could be a good choice, same low light performance as the D3, same frame rate when used with the grip, the grip can be set to accept the D3 batteries, smaller body when not used with the grip (good for everyday / travel use), and used prices have been dropping dramatically since the introduction of the D800/D600.
- Ergonomics are important (at least to me), and it's always a struggle (again for me) to deal with two different bodies when covering an event: different controls, etc... lead to errors and lost shot opportunities. So my recommendation, if you can afford it, would be to have two bodies with very similar controls (D3 + D3s for example).