I think poor old Erwin made a rod for his own back with allowing users to choose between two compression options.
When the current version was first in beta, the names used were Long Movie and Short Movie, the intent being obvious. There was some debate about whether both options should even be released at all and if so what they should be called. Erwin favoured the current Constant and Variable, but Rene thought otherwise and that Constant, at least, had a negative feel to it, that people would associate it with an mpeg encoder's constant bitrate property.
I thought the terms 'Extended' and 'Regular' (mainly for our friends from the USA) were sufficiently obfuscating;) and that moreover some direction had to be given as to when to use one or the other.
In the end, Erwin's preferences prevailed and it was decided to keep recommendations to the minimum, partly because people seize upon them as if they were holy writ rather than guidance, and partly because it ultimately depends on the quality of the original [studio] encoder.
If you get hold of a DVD with high quality encoding then Constant is probably the way to go, especially if the main movie is > 2.5 hours, which is probably the upper limit for Variable mode. But again, how do you know...? Unless you try both...
Finally, for technical reasons that needn't concern us here, the early parts of some [small] titlesets could, in Constant mode, become blocky. I've seen this with, for instance, the 'Metrodome Distribution' logo on the Donnie Darko DVD. It isn't there in Variable mode, but in any case, does it matter - it's just a bit of trademarking before the main feature? You may think not, but there are a lot of angst-ridden people on this forum...
So, once again, I have to insist that there is no True Way. It all depends - sorry, but there we are.