What missions should we play?

A bunch of friends and I are getting back into 40k after not playing for several editions (6-8th) and we are trying to figure out what missions we should be playing.

I have heard people on Tabletop Tactics mention Pariah Nexus but I can't tell if the Chapter Approved Deck is what we want or the Pariah Nexus Crusade book.

Any advice would be helpful, especially if it explains the difference between the two.