As with anything, there are tradeoffs.
Keeping the engines fixed means you don't have to design or cetify an engine to operate vertically or at intermediate angles. You also don't have to rotate such a large wieght, which allows you to have smaller nacelles, lighter rotating mechanisms, have a wider field of view from the sides and more ground clearance. You can also probably translate the proprotors more quickly.
On the other hand, with fixed engines and translating proprotors, you have a more complicated (and heavier) linkage between the power source and the proprotors. The safety crosshaft may be more complex in its totality. Your rotating mechanisms have to be more robust, since they are more complex.
You pays your money and you takes your chances.
Interesingly enough, Chitty-Chitty uses the original Boeing concept in their Tilt-Rotor proposal for LHX, with the engines in the central fuselage connected by shafts to rotating proprotors at the wingtips. Even more interstingly, the V-280 uses the tilting concept Boeing proposed for their version of the aircraft that lost to Belll for the XV-15!. I guess it really does all go 'round in circles!
Last edited by Commando Cody; 22nd Apr 2013 at 19:02.
Reason: futher info