Social Aspects of Photography
Borrowing a lens imposes social obligations that may affect the way you shoot photographs. Borrowing an expensive lens is even more prone to doing so. For example, is the photographer willing to walk around with a borrowed lens on the camera in slightly adverse conditions or take it out of the bag when there is a chance of adverse conditions.
Long zooms are useful for capturing particular images. They also require more time and effort to compose and shoot. In a group setting having people wait around while the photographer zooms in and out to take a particular picture (never mind swapping lenses) may win the photographer friends but probably won't unless it is a group of photographers.
Since you are undecided anyway
My advice would be not to borrow either lens and use the 18-55mm lens that you already have. It's a great range for taking pictures of landscapes and people. Its technical limitations are unlikely to be the limiting factors for your photographs.
The lenses you are considering borrowing exchange weight and bulk and expense for some improvement over your existing lens. In theory, you might take advantage of some of those technical improvements like a wider maximum aperture. Other technical improvements like weather resistance you can't with your current camera.
The 70-200mm adds some reach. It might bring subjects like wildlife closer. It won't bring them close unless you're already physically close. Getting physically close means walking around with the lens hoping to get lucky or waiting patiently for wildlife to come close. Those things appeal to some photographers and not to others.
A travel story
This is a "walking around getting lucky" shot taken at 300mm with a 55-300mm kit lens. I'd shot upwards of 3000 images with the lens prior to taking this particular shot:
The distance was about three meters and the subject was a honking big raven. You can see how close I was when shooting out the window of my truck in the detail:
The lens is less than 1/3 the weight of the one you are considering and considerably less bulky. That's why it was an easy decision to put it in my camera "day bag" so that it was at hand. That's why it was easy to swap onto the camera and hand hold. Because it was my lens, I had plenty of practice swapping it.
I had already taken this image with my 18-55mm kit lens at 55mm:
My first shot was this one with the 18-55mm at 18mm:
The close up shot at 300mm could have been taken anywhere including my backyard. The shorter focal length shots tell more of a story. At 55mm there is a story about a raven on a stone by a lake. The 18mm shot is a rich story because it has other important characters besides the raven.
If you believe you need a lens to shoot in low light, consider an inexpensive nifty fifty. They are small and cheap (particularly used) and will fit in a coat pocket. Yet provide very high technical quality.
If you believe reach will be useful, get a zoom with a maximum focal length of 300mm. The Nikon 18-300mm VR has a really useful range. It weighs about 1/2 of the zoom you are considering and will enlarge objects to cover 2.25x the pixels (1.5 x 1.5).
- Most of what makes a great image is not the lens or the camera. It is the photographer. Being relaxed and understanding one's gear helps.