I've read so many reviews on these two lenses. Many ppl say that the 17 isn't very sharp. However, the reviews don't jive with that. Every review I've watched and read from credible sources says it's extremely sharp. The reason I would consider it over the Pan 20 is the fast focus. The 20 fits my budget better and has a good reputation. But I have little kids, like street photography, and I'm just worried about the sluggish focus. Any thoughts?