my advice to you -
1) dont just go for convenience and get one lens that covers the entire range 18-200. it can never be as sharp as having a prime lens at 24mm, 35mm, 50mm, 85mm, 135mm.
2) 18-200 covers a wide range but ask yourself how often is your most commonly shot range? it is probably just an assurance that you have the range covered but having a wide range doesn't mean you will use the entire range. i use 24-70 for almost everything. if i need to zoom, i can crop from my 24-70 with no loss of resolution.