Some people look down on Mythus Prime, as if it were the bastard child, the very discgrace of the otherwise perfect Mythus.
It ain't so.
Mythus Prime is a more-perfect distillation of the core of the Dangerous Journeys system than Mythus is. The Advanced rules are intended to be encyclopedic, and they are. They are considerably more complete than the Prime rules, and certainly more satisfying for a long, committed campaign.
They're also a lot more work.
I'm not saying that the Prime rules are superior, just a different tack on the same system. If you have the Mythus Prime book as it was published, rather than as a small, unintegrated section at the beginning of Mythus, you are well-set to introduce people to Dangerous Journeys and handle those occasional pick-up games!
I run a Mythus game, using the Advanced rules pretty much as they are written. However, I also keep the Prime book handy. It is the system I use for the sporadic games with my in-laws, and it has proven a great and easily-adapted system for quick pick-up games (ranging from a bug hunt to a quick rescue of a baron's son), and without the time, energy, and creative investment a Mythus HP entails.
Oh yeah, and it's less overwhelming to a novice JM (such as, for instance, my wife, whose only attempts at GMing any games so far have been with Mythus Prime).