I'd like to inform you about my experiences concerning this topic while backing up the Buffy series. Actually, I just have done 2 DVDs up to now, but here's what I found:
I did first the full-disk backup of the 6th disk of the 3rd season. Here, there are only 3 episodes, plus about 25 minutes of extras.
While the difference was subtle and I doubt you'd see a difference on the TV screen, the monitor revealed a more steady and crisper picture in Constant Mode. Especially fine structures like close-up fabric seemed to go slightly in and out of focus every other second when using Variable Mode.
Then I did the 1st disk of season 1. This time 4 episodes on the disk. Lo! here Variable beats Constant Mode!
Variable Mode is less blocky. You can see that especially well in the opening credits where there are many fast scene changes and with the inserted credits that also come out relatively blocky in Constant Mode.
Maybe it's about the same when view on The Great Blur aka TV...
Note that especially the 1st season has a really bad, grainy visual quality on the original DVD. That may be part of the problem.
Anyway, this again proves that Constant Mode isn't always the way to go, even if the movie is 4 x 45 = 180 minutes long!
I wonder how a full 4 episode per DVD of the 3rd season behaves...
Up to now at least for season 1 Variable Mode is the ticket!