From those two choices, I'd recommend the EF-S 15-85mm lens. You don't mention indoor uses, or portraits, and for most nature/landscape shots the slower aperture of that lens won't be a huge issue.


The EF-S 17-55mm f/2.8 is another excellent general purpose lens - I love mine!


I'd stay away from the 18-200mm superzoom - the most noticeable optical issues with that lens are distortions, and you'll see them just as badly with an 8 MP sensor as with an 18 MP sensor.