I can only help with the 120-300. If you never had a fast long lens before you'll be amazed by the size and weight of it. But it is very usable and you can handhold it for short periods of time, even though I wouldn't advice it just for the lack of IS, even tough I did have a few keepers while handholding this lens.
These are a couple of 100% crops of handheld shots of a female blackbird.
f/2.8
f/3.5
These are unprocessed, no sharpening at all. Sorry about the noise, I did apply a bit of noise removal on the background for the final photo but I'm uploading a completely unprocessed photo just to show how the lens performs. THe 3.5 photo is a beauty with just a little bit of USM.
Even though I do like this lens a lot, and i managed to get some photos i would only dream of before (check out
my flickr account) my suggestion would be, if you can afford it, to go for a long prime. Don't get me wrong, the lens is great but with wildlife, and especially small birds, you can never have enough reach, and I keep the 1.4X teleconverter always on my lens. I am saving money to get either the canon 500mm, or the 600mm and I am actually thinking of checking out the Sigma 300-800 mm.
Dont know if Nikon does something similar to the Canon 500mm f/4 L, but if you intend to do a lot of trecking, the combination of weight and reach oo that lens makes it a no-brainer.