Something you might also want to consider is the way the lens renders the "blur" of the out of focus background, known as "bokeh"
http://en.wikipedia.org/wiki/Bokeh
for this you simply have to do lens test as different lens with the same spec will have different bokeh. I think it is cause by the shape of the aperture blades and mainly shows up in highlights in the background such as sunlight reflected off foliage.