The 24-70 is a great lens and no mistake, but its a bit heavy for a general walk around lens. The difference between 18 and 24mm at the wide end is probably more than you think too. On a full frame camera this lens comes in at the 18ish-50ish mm range, which is why people generally say its more useful on full frame.
The 18-200 is light, a good range of focal length and reasonable quality optically - although not great. Perfect for general photography of family, holidays etc where stunning image quality is of less importance to carrying around either a big bag of different glass or something more limiting focal length wise. Thats why Nikon make it...
My other suggestion to you is the same "general purpose" glass I use - a Sigma 18-50 f2.8. Its a constant f2.8 aperture lens and can yield some excellent results and being f2.8 its suitable even in murky conditions. Quite "cheap" too. Obviously at 50mm at the long end but see my earlier comment about the 24-70 on full frame ;-)