You cant look at lenses as having a certain zoom ratio. Hell, prime lenses are, stricly speaking, a 0x zoom.
In SLR terms, the focal length "mm measurement" is an indication of how much magnification the lens provides. More simply: a smaller number means a wide-angle, a longer number is a more distant view.
Both those lenses provide the same magnification: 200mm is 200mm. The difference is that the 18-200 covers a range from wide-angle to telephoto: the 70-200 is a telephoto to telephoto. Your 18-200 will have a 70mm point to it.
The lenses also serve very different roles. The 18-200 is a versatile all-around lens, but the image quality is lacking compared to certain other lenses. The 70-200 is one of Canon's Sharpest lenses and is a telephoto only, so you'd be missing out on the wide end. That's where another lens would come into play.
My gear is a 10-24, an 18-70 and a 70-300. The 10-24 and 18-70 overlap (from 18-24) but otherwise I have to switch lenses to go any longer than 70mm.
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