The games control similarly in that they are first-person action games where the shooting and melee mechanics are affected by cRPG-like character stats and they both feature rather extensive user interfaces for handling your inventory, mission objectives etc. Beyond the overall method of control and how you develop your character to allow for personalised playstyles the similarities end (except for perhaps in how both games are among the best ever made).
SS2 is "just" a shooter where you go wherever the game needs you to. Almost all exposition comes from audiologs conveniently scattered about, though only some contain vital information on how to progress the game (keycodes for unhackable doors mostly). The game plays as fast as you want it to because of the simplicity of the story. The design is simply a very tightly packed game with economy in every aspect. Everything is simple and straght-forward. Take away the character building stuff and you have exposed it for the well designed shooter it really is
DX on the other hand is much closer to an RPG in that you have to engage in conversation with many characters throughout the game (with dialogue options). Character interaction and the (storywise) developing relationships are the meat of the story, and while they provide almost no gameplay difference, the story has branching paths with dramatic turns of events in the game universe (major characters may live or die depending on your choices for example). You are not just tailoring your character in DX, you are tailoring the story and affecting the people of the world by your actions. They are not deep choices, but the game revolves very much around the illusion of them being so. Behind the guise of an engaging RPG lies a rather simple shooter with simple stealth mechanics. It is definitely the story and the setting that makes the game a superbly enjoyable experience. The game does offer a spectrum of playstyles ranging from complete stealth, non-lethal takedowns, "flexibility", silent killer and straight up terminator killing machine. It doesn't actually matter, but it makes the game more enjoyable if you roleplay what kind of cop/spy you want to be. Gameplaywise it's a shooter with strong emphasis on sneaking or taking out guards without alerting everyone. I feel that the story and characters are in favour of spilling as little blood as possible, which stands in contrast with SS2's survival story where you do whatever you have to in order to survive a disaster.
The combat in DX is a bit wonky. I'd say SS2 is the better shooter of the two. In SS2 you are fighting monsters and bots, for which the AI is ok, while DX is almost entirely about fighting or sneaking past human opponents where the simple AI is ruining the illusion a bit. DX cleverly distracts you from this short-coming by gently imposing a sense of guilt for starting big shoot outs or cutting the spine of every guard you come across, the dumb AI allowing you to sneak by with ease and not having to kill some dude who is just doing his job helps sell the story.
The real argument as to why you should progress from SS2 to DX is that you obviously have the patience for these games, and one thing both games have in common is that they respect you as a player and treats you as an attentive adult who don't need a back seat driver telling you what to do every second. You are going to spend a lot of time with both games, perhaps even get lost a few times, and that is perfectly fine. These are games for adults in a real sense (not in a sex and gore kind of sense).
