I've got two lenses for my Nikon DSLR (the D5100) - the 18-55 zoom it came with, and a 55-300 f4.5-5.6 zoom that I really enjoy for wildlife. The problem I'm running into, is that I often find my self in situations where I'm swapping back and forth between the two lenses which is (1) risky and (2) annoying.
I'm thinking to keep the 18-55 zoom for everyday use, and sell the 55-300 and get one of the 18-300 zooms. On nikon.com there are two flavors of 18-300 zoom, the only difference I see is that maxes at f6.3 and the other at f5.6.
The one with the dimmer aperture is the newer one. The main difference is that it is built to be much lighter at 550g vs 839g. It also has a newer stabilization system which is rated at 4 stops, vs 3.5, so that balances against the aperture difference.
There is no point keeping the 18-55mm once you have the 18-300mm which is in fact a better lens optically, although it is an ultra-zoom which compromises on ultimate quality in favor of versatility.