Qimage will send the image at the resolution shown. So if you set it to 360 PPI, it'll send the image at 360 PPI. If set to 720 PPI, it'll send the image at 720 PPI. Unless your image happens to already be the resolution shown (at the given print size), it'll resample. If it didn't resample, the driver would do it for you... and you want Qimage to do the resampling and not the driver. Qimage has higher quality resampling routines.
Mike
I believe that Mike gave you the best explanation possible. See above.
What he is saying is: If you set the driver to a quality setting that asks for 360 ppi, Qimage will interpolate the image and print size you gave it so the driver will get 360 ppi.
If on your Epson, you checked finest detail in the driver, depending on paper and quality settings, the driver will ask for 720 ppi. Qimage will interpolate to 720 ppi. You might or might not SEE a difference.
You always want Qimage to do the interpolation; up sampling/ down sampling as the quality has been unmatched for 20 years.
Fred