I have both an RZ with 50ULD, 110, and 180, and a 645 1000s with 45/2.8, 80/1.9, and 150/4.
If I were going out to shoot landscape, I would not hesitate to bring the RZ. The 645 is a handheld or critical weight camera for me, and I really only bought it for the 80mm.
edit: That said, the difference in size between the two is astonishing. The 645 isn't much bigger than a modern SLR, and the form factor helps a lot. I don't carry it with a grip.