Compare the Zeiss with the Sony 16-50mm. I can't find it but I recall a comparison by David Kilpatrick in which he found both excellent, but such a different combination of vices and virtues that he decided to keep both. One little remarked advantage of the Sony is that with the latest versions of Sony's IDC RAW converter you get automatic lens correction (geometry and chromatic aberration), also in-camera jpeg corrections with later Sony A-mount bodies, but not with your A550.
I have the Sony 16-50mm, and use it for a walkabout lens in interiors, evenings, or dim winter daylight, but for an outdoor walkabout lens in good light I found to my surprise that I prefer the old Sony 18-250mm. Its IQ defects are much less noticeable when shooting at f8, my usual good light default for opportunistic walkabout grab shots, and the instant accessibility of the longer focal lengths trumps the bother and lost opportunities of lens switching.