Hi Art,
My concern (and maybe I shouldn't have a concern since my prints are great) is the appropriate ppi setting to use in ACR - from reading on this forum, it seems I am interpolating once to 300 ppi in ACR and then to 600 ppi in Qimage. Is my understanding correct? If so what setting would be the most appropriate to use in ACR?
The ppi setting in ACR is merely a tagged value and in some sense meaningless. In real terms, an image file cannot have PPI, pixels per inch, because there are no "inches", just pixels. Prints have ppi but 300ppi is not necessarily the most appropriate value.
So, there is no need to be concerned about that tagged value, just use the image as converted, at it's full pixel size with no re-sizing in PS.
Qimage is interpolating to 600ppi because that's the "native" resolution of your printer (a Canon or HP with that value) and you get great prints because Qimage does that with very advanced interpolation algorithms and print smart sharpening.
Did you see Mike's latest article on smart printing here?
http://ddisoftware.com/tech/articles/march-2010-smart-photo-printing/A timely reminder of why Qimage gives you great prints
Terry.