Just discussing the benefits to having multiple lenses compared to one long lens. If I'm not mistaken, Mike prefers quality over convenience especially on an outing specifically for photography. Based on that I'm leaning more towards keeping both lenses and upgrading as I go. The 18-55 isnt the greatest lens but it will do for now. I started out questioning whether the 18-200 is better than the two lenses but if the quality is noticeably worse than the convenience isn't worth it. I'm usually on hikes and random walks where having one long lens would be worthwhile but I guess I'll just settle for sticking with a mid lens and finding the shots I can make work with that lens. I really should get a prime lens and force myself to compose without a zoom.
I try, although not nearly successful enough, to go by this advice I read somewhere . . . . . . .
"Don't try to decide which lens I need to get this shot, but how can I get this shot with the lens I have".
What I guess I am saying is don't go buying lenses to improve your photography if the lenses you have will do the job just fine.
Still, there is a validity to what your concern is. You want a lighter and more convenient package to carry with you on your hikes. Hikes must be the primary time for you that you do your photography. If so, then you should strive to get what you need out of your gear, be it light weight and convenience or quality. Try to get both. Either of those lenses, 18-200mm or the 18-300mm, may work great within the ranges you use them for what you do. If you find you mostly use shorter focal lengths and into the wide angle lengths, get a zoom that more closely covers the focal lengths you need the most. that way you get a much better quality image out of a lens that offers the focal range you use.