An adapter is great if you already have Canon lenses or want to share lenses between Canon DSLRs and NEX cameras - but it isn't that good compared to getting the Sony lens.
No adapter is perfect (actually, the lens and camera aren't perfect either)
The adapter will add some more (usually small and almost unnoticeable, unless you pixelpeep) misalignment to your camera and lens existing (hopefully small and unnoticeable) misalignment - it will always make the optical quality a little bit worse.
Also, it might but doesn't have to (don't know about the specific model you plan buying) make auto-focus slower and/or less accurate.
Add to it that the Canon EF-S 18-135 (that I have and love) and the EF 18-200 are both "kit quality" lenses and are, how to say it gently, not optically the best lenses Canon makes.
Also the Canon 18-135 and 18-200 are big and heavy compared to NEX lenses.
So, you are taking a lens that isn't that great to begin with - and that's too big and heavy to be used comfortably with your camera - and add to it an adapter that will make it worse and even slightly bigger and heavier.