A few years ago I purchased the Rogue Trader and Dark Heresy RPGs from Fantasy Flight Games. These games are both set in Games Workshop's Warhammer 40K universe, and share a common system that is a close relative of the earlier editions of the Warhammer Fantasy Roleplay system. Both books are very well produced, as expected from FFG, and are massive RPG tomes topping out around 400 pages each.
I have played both games, Rogue Trader far more than Dark Heresy, and I found the system to be mechanically sound in each. There are some variations in character creation between the two games, with Rogue Trader taking a more linear approach, while Dark Heresy uses a branching system. Both games use a percentile based task resolution system, and take a fairly traditional, GM, Players, roll to hit approach to gaming. There are some light "story" mechanics in both games, but traditional gamers will feel at home in both.
Dark Heresy is kind of a depressing game. In the '90s there was a push towards darker, grittier games like Shatterzone, but this is a completely different animal. Dark Heresy is about hunting down aliens, mutants and heretics. You are a servant of an emperor whose rule is only slightly more justified than Emperor Palpatine's from Star Wars. The game takes an extremely utilitarian moral view, all of the horrible things done by the Imperium are justified because it defends humanity from far darker things. The future is soul crushing, most people die where they were born, doing what their parents did, with no hope of escape. Millions of people are sacrificed daily just to keep the Emperor alive, he is basically a corpse feeding on the souls of others. And these are the good guys. This game does not present this tongue-in-cheek, it is played completely straight. And that was a turn off for me. While mechanically this game is fine, the setting is awful, why would anyone want to pretend to live there.
Rogue Trader, while technically in the same setting, has a different tone than Dark Heresy. Rogue Traders have much more freedom than the vast majority of Imperium citizens. Rogue Trader has far more in common with Traveller than it does with Dark Heresy. The game revolves around a Profit Factor mechanism that you raise by taking on Endeavors. You try to open up new lanes of trade, found new colonies, explore old ships for valuable technology, and perhaps smuggle goods. It is a more open, sandbox style game, and you have the ability to chart your own destiny, unlike Dark Heresy.
While I enjoyed Rogue Trader far more than Dark Heresy, I am going to eliminate them both. Dark Heresy for the obvious reasons, and Rogue Trader because I already have so many games that do the same thing. I have several versions of Traveller, Edge of the Empire, Shatterzone, and GURPS Space to cover this kind of game with pretty good variation on the theme. I just don't need another one, especially when I am not especially fond of the setting.