I have read several reviews for each of these lenses, but I'd like to hear from folks who have actually used both. How well do they perform wide open? Is there really a noticeable difference in IQ (especially if both lenses are stopped down to f5.6)? How well do they focus, etc?
I own both (f/4 for compact travel, f/2.8 for everything else). They are stupid sharp wide open, and focus speed is excellent. The only downside of the f/4 lens is, well, f/4.