Howdy, I was thinking about which to buy before and ended up with the planar (not the popular choice it would seem). The Planar is the sharpest, strongest rendering lens I have ever encountered. It has great presence when shooting color and it lets you know it so you have to be able to control the sheer power of such a lens. The sonnar, as with all the sonnars I have ever used are lower contrast, also the images lack the sharpness bite to them, which is fine, just a different type of tool, but many people say the sonnar out of focus rendering is smoother, I tend to disagree, I find my planar to render smoother then the sonnar, in fact in my experience with sonnar's on hasselblad's, m42's and M mounts, sonnar's tend to render with a "busy" background where lines are generally easier to make out then the planar, just my experience though.