There are two V6s - petrol and diesel.
I wouldn't get a petrol, there's a chance I might want to do a river crossing (have done), so that leaves my choices between the 4cyl and 6cyl diesels.
There's a significant cost difference. The V6 diesel does have more power and although the engine is slightly larger in capacity, it uses less fuel to do the same work as the 4cyl engine - it seems it would be about 1-2LPHK roughly. That's (let's be generous) about $3 saved for every hundred km.
To recover the $20,000 price difference that I'd have to fork to upgrade (and that doesn't include the fitting out of the extra gear) I would need to drive the vehicle for (20000/3)*100 = 66,600 km. I can do that in two years ... but don't forget the rest of the gear I have to replace. It'd be closer to 3 years before I recovered the upgrade cost.
That said, if money wasn't a consideration, I'd go the V6 - but not yet. The current V6 diesel has heat issues and I don't want to have to fight the car - or Nissan.