I had a long think about it and I am going to say, go with the Emperor's Shadows. For two reasons...one, they seem to suit Primarus Marines more. The Carcharodons seem to be all about close assault and so far the Primarus Marines don't seem to have that going for them. And two, there is so little information on the Shadows you're free to fill in the gaps yourself. It was a tough call though because they're both awesome chapters.