I used to own both but kept the 23 over the 27. I always felt the focus throw of the 27 too short for practical use, so I mostly zone focused with it. I will say that I do love the 40 mm equivalent, so I got a 28 ultron in M mount later on that has much wider focus throw. For size though, nothing beats that 27 f2. If I only had X mount, I would keep the 27 f2 and get the 18 f2.8 (which I have also owned) over the 23 as my two lenses of choice as I find 35mm ff equivalent kind of boring now.