The impressive part was that the comparison of the two images was taken from a very tiny section in the far distance. I focused somewhere in the middle and got practically everything in the image quite sharp. Attached is another larger and closer crop, and again PTGui is a way sharper. In all fairness, PS CS5 is now about 10 years old, and PTGUi version 11.9 has been released just this month, so that may play a role, too.
I am very pleased also with the Canon 20mm/2.8 lens. It is very sharp, relatively light and about $250 cheaper than an equivalent Nikon lens.