Nibali winning the Tour is extremely unlikely: Contador is superior at both climbing and TT'ing. And even if Contador wouldn't compete, there's so many GC riders who climb much better than Nibali: Schlecks, Evans, Sanchez, Basso, Menchov, probably (though I hate to admit it) Gesink and VDB. With enough TT kilometers and perhaps some downhill finishes he could probably beat a few of those, but it will be very hard for Nibali to win the Tour. He might have a shot at the podium, but he will have a hard time against the other contenders and will need quite a bit of luck.
In the Giro however, Nibali could be one of the strongest. Scarponi he can probably handle if the route is easier than this year. Other than that, it will highly depend on which other riders go for the Giro. He definitely has a big chance of winning the Giro though.
If I'd be the boss of Liquigas, I'd let Nibali lead at the Giro again while Basso takes on the Tour. As said by others though, the parcours and other variables will need to be taken into account before making a decision.