Well, the image quality might not differ very much, but with the PDI card you can much more accurately adjust the capture settings. This is very helpful with the sharpness settings. To reduce the ghosting the sharpness has to be reduced. When the ghosting is at a minimum the image of the PDI is slightly sharper.
Although I don't have the old captures anymore I found some screenshots I once mailed. But they are kind of difficult to compare because the PDI captures are much brighter. Anyway, here they are.
MCE TV (7135):
Sweetspot/PDI deluxe (7118):
MCE TV (7135):
Sweetspot/PDI deluxe (7118):