27mm vs. 23mm is a straight choice between size and sharpness. Had 3 day trial with the 23mm but bought a 27mm without testing because I was also getting a 14mm. 23mm wins hands-down for sharpness and colour but love the size of 27mm for a relatively discrete walkabout. Still, if money no object I'd buy the 23mm as well because it is IMO a superior lens and not too large either