Second body for wildlife photography

Without a doubt your best bet would be a D800 or D600.  You'll get D4 level high ISO ability, DX crop mode, and DR that's improved over your D300s.

A D7000 will give you perhaps 1/3 EV better high ISO performance over your D300s, but the two FF bodies will give you 1 to 1 1/2 EV better high ISO performance.   A big difference and more than worth the added expense.  A D700 (provided it has a DX crop mode) would also be a good option, but I think you'll be happier with the improved 100% VFs of the D800 or D600.