What is the difference between Tilt-rotor aircraft and Tilt-wing aircraft?
I would say that the complexities of tilting the entire wing are what compelled Bell to choose just the nacelles.

By complexities, I am thinking 2 major systems. The first is the flights controls. Based on the how old that image looks, I suspect that aircraft want designed with a fly-by-wire flight controls system if it was even an option. That would be, in my head, the only simple way of managing the flight control sister's. Otherwise the designers would need to get VERY creative with the pushrods or cables &amp; pulley systems. That would be an engineering nightmare!

The other system would be fuel. Engineers are going want to put fuel in the wings, as opposed to under the floor in the fuselage. That way cargo space is not used, bit more importantly, that much weight centered on the shortest works reduce the ability of the aircraft top perform missions for to the Zero Fuel Weight considerations. 

Again a tilting wing creates some engineering challenges they may just want to avoid. The wing tanks would shift weight as they tilted back, and they would need to ensure fuel could still get to the engines. That's not even the hard part either. Think about trying to run the cross-feed lines through a rotating wing root! No thanks!!

Hope that helps, that's all I've got!!