I have the DA16-45, as well as the DA21 and the DA40. They are three of my favorite lenses for travel.
To me the big difference between the 16-45 and the 21 is the size, weight and bulk. As much as I like the convenience of a zoom, after a day of carting it around I always seem to switch to the much smaller prime for the rest of a trip. I carry my camera in my hand at my side often, after a bit any zoom gets heavy.
Here is a gallery of shots from a short weekend trip to San Francisco. The first day was shot with the DA16-45, after that it was all with the DA21. If you mouse over the larger image on the right side of the smugmug page a button will pop up that will show allow you to see image data, including the focal length. You'll be able to tell when I switched from the 16-45 to the 21 partway through the trip.
San Francisco Trip
I shoot handheld, and never shoot test targets, so any minor differences in image quality between the 2 lenses is lost on me. In the end I think it comes down to what feels better in your hand when you're walking around sightseeing.
By the way, I've been using just the DA40 for a while as a walking around lens. With a bit of care it really is pretty versatile.