I would rather suggest you to try more with what you have. A lense with more variations like 18mm to 200mm will have more elements. A lense that is having lesser elements would have less compromising factor. Have used to with these lense and the scenario of switching lense is as same as told by Dan. I would go for 2 lense rather than the one but that will have a problem of switching lenses. Now it is upto you :)

I would suggest you try to do more with what you have. A lens with more with a greater zoom (such as an 18mm to 200mm) will have more elements. A lens that has fewer elements would have less compromise in the design.

Have used to with these lens and the scenario of switching lens is as same as told by Dan.

I would go for 2 lenses rather than the one, but then you will have the problem of having to switch lenses. It's your choice