Is there any difference in performance (other than focal range) between Canon's EF-S 18-135 IS and 18-200 IS kit lenses, in terms of sharpness, contrast, AF speed, or anything else?
I can't decide which one should I get with my 60D, and 18-200 is not a cheap lens. Is it really worth the price difference?
All in all it depends on what you are doing with the lenses. The 18-200 gives you more range clearly. They both are the same FStop. The decision really lies in whether or not you actually need the extra range.
My advice is to rent them both from a place like http://www.borrowlenses.com/ and try them out before you buy. Heck, try a few different lenses out and then make a decision. The last thing you want to do is buy a lens and have it collect dust. Also, consider buying used. Photographers take care of their lenses for the most part and they are definitely going to be some deals out there if you look around.
As you have already found, they are more similar than they are different.
One of the most conclusive pieces of information on the comparison between the two can be found in the review of the EF-S 18-135 lens at the-digital-picture:
While its focal length range is shorter than the 18-200's, the 18-135 is less expensive and has image quality that is similar or slightly better in sharpness and distortion.
I would take a look at the reviews of both lenses to see which one might fit your style and price range better:
It is anybody's guess why Canon introduced both lenses in back to back years 2008/2009. I would guess that the price difference is the main factor that many people are considering. The 18-200 is around $600 USD, and the 18-135 is around $350 - so it is quite a difference for most people. I would consider which one fits your budget, and purchase accordingly.
This is a direct comparison of the IQ of both lenses wide open at 18mm: the-digital-picture.com
As others have pointed out in comments, other options may be better, such as purchasing a 70-200mm f/4 non IS, or maybe a 50mm f/1.8. The options are endless, but I'll keep this answer just focusing on what you are specifically asking.
To me, the 18-135mm is a bit better IQ at a lower price point, so if I was only looking at these two lenses, I would go with the 18-135mm.
