The 18-200 is not a good lens for architecture: its got some very complex distortion thats near on impossible to correct.
The 18-70 has some distortion as well, but its easily corrected if you really need it to be.
Ideally, you'd use a PC-E lens: the perspective-control mechanisms allows you to keep verticals vertical and yet maintain the proper perspective. The downside is that theyre super expensive.
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