I have a Pentax 67II, a Super Speed Graphic, and a Rolleiflex, and I've used them all for landscape photography.
I love the portability and ability to hand hold with the Rollei. I also like square images, but if you shoot more rectangles I'd go with a 6x7 rather than crop.
The P67II is a breeze to use in the field. The body, 2 lenses, and accessories do fit in a slightly smaller backpack than my Speed Graphic set-up, but it's not much smaller. I'll use the P67II hand held for some portraiture, but overall I get much better results on a tripod when it comes to landscapes. Roll film is easy to pack in the backback.
With the Speed Graphic I have to carry an extra film bag. I can cram a dozen film holders in an insulated lunch bag; I have yet to actually go through 24 shots before returning to camp/home/darkroom. I regularly hike up to 4 or 5 miles from my car, and overall the P67II rig doesn't seem much more portable to me. Both set-ups are heavy!
I'm pretty sure I can see the difference between prints from my 6x7cm and 4x5" at 8x10, and I know it's there at 11x14. Also I like the options that even the limited movements of the Super Speed Graphic give me. I'm pretty slow and deliberate with landscape photography, so I don't find the operation of the P67II to be any faster than the SSG, although I do tend to take twice as many shots when using roll film.
I shoot BW most of the time, and do my own processing. I would have to mail out 4x5 E6 or C41 for processing, while I could get 120 or 220 processed in town. So if I need color I go with the P67II, but for BW I'll choose the SSG. Someday I'll pick up a roll film back for the SSG, and the P67II may never see the woods again.