I haven’t heard much about the 120-400 – most people seem be prefer one of the 500mm zooms.
The 120-300/2.8 does pretty well as a telephoto landscape and focus is close enough to get some nice close-up work. The OS version is getting an update so you may look for a used one. When I got mine I had found a good deal on Amazon that was about $700 below other retail prices. It can be worth searching around for a good deal. If you find a great deal and are not sure about the retailer come here to Nikonians and ask about them before making a commitment.
Dave Summers Lowden, Iowa Nikonians Photo Contest Director
Nikonians membership - "My most important photographic investment, after the camera"