30K: Burning of Prospero vs. Betrayal at Calth

We've already talked about what you need to start playing 30K. Here's a tl;dr: 2x Betrayal at Calth. But now GW has released another boxed set based on the Horus Heresy, or, more precisely, the Burning of Prospero.

So how does Burning of Prospero compare to Betrayal at Calth? Which one should you get, and why?

Let's contrast and compare.