Mokh, I think what may not be obvious to you at the moment is the different field of view you are going to get using the 5DIII over your XTi.
Your 18-55 on the XTi was effectively a 28-85. Therefore the 24-105 with the 5DIII is going to be the lens you want for the Honeymoon. The 24-35 range will give you a great wide shot for Landscapes and the 85-105 range will capture great portraits. You will probably find yourself zoomed out or in, much less in the middle.
The speed of the 16-35 is certainly an advantage in dusk/dawn situations, but I think you will quickly realize, looking through the viewfinder, that it is not your go to lens for the use you describe. Besides, the 5DIII really shines in low light situations with the 24-105.