Humminbird- with straight Di it is going to give you a better picture than Si/Di unit because they blend the two Si shots into a Di shot using the internal computer. Now I think but not positive but on the new high end mega units they may have a separate Di ducer on the Si.

Lowrance uses a separate transducer for their Di VS Si. And that ducer works across a broad range of units. So I would figure the quality would be more of a function of the number of pixels in the screen. Lower end unit less quality.

Garmin I have seen seem to have the same quality of picture spread across their entire line.

I would expect to have a little less quality from the lower end units than the high end units.
But if your talking about the same unit ( Lowrance hook2 7 ) one with gps and one without and that price difference then no you would not suffer on quality nor on any other unit.