The main gain with DVDs for me is avoiding CD swapping. Image quality may only be marginally better and on some systems not really noticeable.
This is my primary motive in preference for the DVD version of many games.
As for differences. I don't think the graphics were very different in the DVD Atlantis 3/Beyond Atlantis 2 - but I did notice that I had little of the glitch errors that many others ran into with the CD version. I never used a patch or anything with the game.
Schizm is one game that was intended for DVD only - but they made a scaled down CD version to make it more marketable. I was thrilled with the DVD version of this game.
On Dark Side of The moon - I think playing it on DVD is almost a must to really appreciate the game. There is CD swapping to the extreme in the game and it can ruin the experience. (Every time you enter the elevator you have to swap CD's and then back depending on where you go.)
The rest I am not sure about.