I think these two are not to be compared in the way you`re trying to - it`s not the same camera "line" (despite the "S" name), where S200 is an offspring for people that don`t need all the options (including the image quality) brought by the regular "S" models (currently S120), and yet would like to pay less for after all a more modest package. And I think they both appeared almost simultaneously, or S200 maybe some days/weeks later, but S120 is still more expensive, and a more capable one.
S120 is the latest (last) iteration of a line that includes S90, S95, S100 and S110. A successor here is still not even being rumored about, and some even doubt if there will be a successor at all, after G7 X has been made - but while surely much better, it is still much more expensive (and while still pocketable it is noticeably thicker/heaver), so it may not be a real alternative for people being happy with an "S" PowerShot.
Here`s a quick comparison between S120 and S200 --
http://snapsort.com/compare/Canon-PowerShot-S120-vs-Canon-PowerShot-S200, go through it, noting all the differences between the two (if not already), and while it really depends on personal preference and needs, I think that S120 is (much) better overall.
Regarding the image quality, S120 should come on top, especially as the amount of light in the scene starts to drop ((back-side illuminated?) CMOS sensor vs CCD), and having a possibility to process RAW files should be a plus as well.
Please note that some will argue that CCD sensor (found in S200, and previously in S95 before it was replaced by CMOS in S100) will produce better images in good light, yet I can`t comment on that (no much experience with it, really), but I doubt the difference (if any) would be all that obvious (except maybe on heavy pixel peeping, which these cameras are not made for) - please someone correct me if I`m wrong.
And one more thing - landscape photography usually requires sharpness from "end to end", where it is a huge challenge for a majority of small, compact cameras with zoom lenses, especially at the wide end on the lens (where landscape is usually shot with), as additional software correction is needed to compensate for image distortion, causing a loss of detail (softness, smudginess) in the corners of the image.
This is to be expected, and many may not even notice it when viewing images on the screen or making a small to medium size prints, but once the image is zoomed in or the print is a bit bigger, it will be noticeable - yet that is the price you pay for otherwise a convenience of having a small, compact camera (with a zoom lens) that fits in your pocket.
