18-300 is certainly an awesome focal range for a simple walk-around lens. It comes with it’s limitations though. If you don’t think you need the wide aperture in low light (ie, you’re using it more for landscape or middle-of-the-day events/wildlife than for indoor shooting), and if you’re not concerned about chromatic aberration, excellent image quality, or barrel distortion on both ends of the focal range, then I think going with the name-brand is a good choice. I don’t personally know enough about specific lenses (especially when it comes to Nikon) to tell you any differently.
For me personally, I prefer to invest in a lens that does best for what I know I’m going to be using it for. I love landscape photography, so I invested in a quality 16-35mm 2.8 that I hope will last for many, many years. I find myself doing lots of event and wedding photography, so I invested in a 70-200mm 2.8 that I hope will last for many, many years. I am seeing a need now more for a better portrait photography lens. Right now I use the 50mm 1.8, but am looking around for investing in something that I will like better. Of course, I am into the business of photography, so quality really matters to me.
So, if you don’t do any one specific kind of photography, or you’re not intending to really get into the business of photography, and if the thought of changing lenses all the time (which I don’t really do actually much, since I usually am not doing landscape/wedding/wildlife/portraiture in the same shoot) dampens your enthusiasm for taking a camera with you, then of course, an 18-300mm walk-around lens is a fine investment.