I mentioned last week that I really dug Original Sin as a collection. However, due to the nature of the tie-ins, they have NOTHING whatsoever to do with the event or each other. Inhumanity at least had the thread of new Inhumans being birthed. On the other hand, it's looking a lot like that Original Sin Annual is going to be otherwise stranded, and I'd wager they squeeze it into the companion.
Of course, if you don't buy all of those other series collections regularly anyway, the Companion is the tidiest way to read a ton of good stories, so that should be your main guide
Personally, my stance is that all Companions get clearanced eventually, so unless they include otherwise uncollected issues from an ongoing I buy (as Inhumanity Companion did) I will abstain until they get cheap and I discuss them in the other thread