Hello,
I think you will not use there 100-300 at all (I maybe mistaken)... I was there 3 times last year (I live in Phoenix, AZ), and every time I took wider lens.. Started with 28mm, then 19mm and finally 8mm fish eye and I would go wider if I could.
Dont know what is your shooting style, but I would say that pretty standart picture for Grand Canyon is half sky / half top edge of the canyon, or 1/3 sky , 2/3 canyon, etc when edge makes horizon line.. In sunny days that's pretty contrasty.. Sky is bright and canyon bottom is deep shadows / almost dark so I would not recommend contrasty film if you want to keep shadows details. Again, it's all based on my own experience. I'm not Pro..
Check my profile, there is link to my picasa albums.. I have some Grand Canyon there.
Vic