Lisa, I think the 70-200mm or 70-300 mm zoom lenses are super-handy, and that's one reason that the 70-200mm zoom of modest aperture (like f/4 maximum aperture) has been around since the 1970's: it's just a great range of focal lengths to have in one, fairly lightweight lens! I own several lenses in this range, as well as an older 80-200mm f/4 Nikon lens.
You will find that for family photos and day trips and just walking around and shooting pictures, that the 70-300mm lens is fantastic when what you want is a selective angle of view. To answer your question, if I had to take only one lens, an 18-55 or a 70-300, which would I take? Honest answer? I like telephoto pictures, small slices of the world, so I would choose the 70-300mm zoom. I personally think that many landscape photo situations look best with a telephoto lens. When I look at the world, I see things in a narrow, selective angle way.
The sunset shot above has a typical problem: black foreground that adds little interest, and a beautiful,spectacular sunset sky. One common shooting-time method to improve this is to use a graduated neutral density filter to even out the scene; the graduated ND filter is like sunglasses for your lens! A four-stop graduated ND filter could have darkened the sky, allowing the dark foreground to be exposed through the lower, clear-glass part of the filter, thus "equalizing" the exposure between bright and dark.
Another way to have shot this? See the plants shot-left? Move 25 feet to the left on foot, then frame those plants up, and silhouette them against that glorious sky! This could have been done with the 18-55mm lens pretty easily, or the 70-300mm lens. As to the exposure, at 800 ISO...the dark foreground is pulling the speed "down", or slow; this could have been shot at a slightly higher shutter speed, if the metering had been based off of the brighter clouds.
The secret in this type of sunset shooting is to find a large object that is close-in, and to silhouette that object against the sky. Otherwise, one ends up with a large, 1/3 to 1/2 of the frame area used on what will end up as black, subject-less areas. In this case, the two telephone poles and wires in the middle of the image could have been used as subjects, with the 70-300 lens zoomed to 250-to 300-mm, and an orange sky being shown around those two objects. The trick is to find a substantially-sized object to silhouette against the glorious sky...and with a long zoom lens, like a 70-300, you have the ability to get low, then zoom in, and MAKE things pop out of the world in front of the camera, by magnifying their on-camera size with the telephoto effect; with the 18-55 zoom, you have a wide-angle to normal lens effect set at your disposal. This is why I choose the 70-300mm zoom for more work than I do a wide-to-normal lens...I prefer the ability to zoom in, to pick things out, and then to focus in on them.
Lastly: the 70-300 zoom offers the potential for what is called selective focus work. Selective focus is like shallow depth of field work. Selective focus is using shallow focus planes to make objects stand out from the foreground or background,and it is something that a 70-300mm lens is very good for at the closer and medium distances of 2 meters to 20 meters.