At $29.99, it's hard not to recommend Retribution. You're getting a standalone experience with an exceptionally large amount of replayability, depending on how into Warhammer 40,000 you are. And even if you ultimately only touch one of the campaigns, try out the multiplayer for a bit, and dabble in The Last Stand, it's worth picking up.
It's also worth considering for anyone who has yet to touch the series. You can absolutely dive headfirst into Retribution and have a blast. In fact, that might actually be the ideal way to do it, as much of the repeat content will feel fresh.