I can't comment on the Voyager discs, but I imagine they look better than TNG too.
The Borg Fan Collection has episodes from 3 series and is a good place to compare MPEG-2 encodes from the videotape masters.
TNG is soft and looks like old video while VOY which was mastered to D-2 videotape format looks the same as the day it was mastered
thanks to digital video being...digital. There is minimal loss of quality on the MPEG-2 encoding itself
from the master tape.
That said Bill Hunt the main reviewer over at The Digital Bits website felt that VOY season 1
picture quality was:
The video quality on this DVD set is generally very good, but not great. The episodes are presented in their original full frame aspect ratio, but they're somewhat soft looking and continue to have a digitally-processed look to them, which has more to do with the way they were post-produced than their preparation for DVD.
I can assure you when watching episodes from the Borg Fan Col. that there is a noticable difference between the two generations of series.
It's odd that when you watch an episode from TNG in 1993 to an episode of ENT in 2003 (Regeneration #049) which was mastered to HD and then downconverted to D2 standard-definition videotape how much the quality of post production of 35mm telecine to video tape, editing, & mastering had changed.
Since TNG seasons were put out on DVD in 2002 I don't think MPEG-2 encoders changed THAT
much between 2002 and 2004 when VOY was encoded for DVD.