I have no experience with either of those lenses so I am not even going to attempt to speak to the issue of the comparisons in their quality. But if you’re looking for a wide angle for landscapes I think you would find that 16mm isn’t going to be much better than the 18mm that you currently have. I use a 12-24mm quite a bit for landscapes and I find myself either at or close to 12mm a lot of the time. So if all you’re looking for is an improvement in image quality over your current lens, the prime 16mm may or may not fit the bill. But if you’re looking for greater flexibility in being able to improve your compositions with the use of a wider angle then I think you’d be better off with the 11-16mm
