I don't have the time to play it and won't for a while. Here's what I can say though:
You get account XP for completing missions. You spend this XP outside of the main game loop on quite a few different things.
First, it's broken down by character class.
Then it's broken down by their weapons (about 6 per character), their armor set, their "Perks" and their banner.
Within each category (except banner), there are multiple things to buy with XP.
Each class has at least 6 armor variants that are a recolor and in some cases a reskin of their terminator armor. There are also perk like "widgets" you can purchase and attach to you armor. These are broken up in to a couple categories, but the general gist is one category is a straight alteration effect like "20% more this but 20% less that", and the second category is a series of perks that usually are like "deal 5/10/15/20% more ranged damage" or "etc...melee damage" or " etc...armor."
Each weapon that a class can use also has recolor and reskin options you can unlock, as well as perk like upgrades you can buy. Most of them add cosmetics to the weapon along with the package, so you can put a snub nosed barrel on your storm bolter to increase damage, or longer thinner barrels to increase accuracy, or a bigger scope for dat 3x aiming on your fully automatic Storm Bolter...There's medallions and badges an stuff you can unlock and attach to the weapon too that don't do anything but look cool.
Each character class also has a set of perks unique to them you can unlock. So the Assault has melee and survival focused perks while the Heavy Weapons guy has ranged-focused perks and the Psyker has psyker-power focused perks...you get the idea.
All in all the cosmetics I feel are fairly robust but the gameplay side of customization....a little weak. A little drab. It's % based upgrades to broad categories like "melee damage" and "ranged damage" and "armor" and "health". "Make this psychic power do more damage." There's the occasional cray cray mod in there too like "attack speed with an axe." While it might help make a certain Terminator build better at what they already do, or cover a weak point of their's, I don't really think they're going to add that much to game except to let you challenge higher difficulties to unlock more stuff faster. I suppose it's better than nothing at all and certainly gives you something to chase, but let's be honest, we've all been playing enough games lately to be getting a little tired of the whole unlock grind thing. And this one isn't especially interesting to boot. I give them props for the weapon customization and the fact they tried to give you multiple (and I do mean multiple) categories to choose improvements and tweaks. Seems to be a little bit of a case of quantity over quality though.
To note, you can only equip one upgrade from each category at a time.
And then there's several different banners you can unlock, which I think changes the banner hanging in the "ready room" of the game.
In addition to that they've added a Interrogator Chaplain class, which I think has vaguely-team-buffy sorts of abilities and gets a Crozius Arcanum and all that good stuff.
In addition to all THAT. some guy from the steam forums said the following:
Enhanced edition brings a TON of replayability in the form of Special Missions. As a closed beta tester for EE I can tell you they ramped up the number and quality of Special Missions over regular Deathwing by an order of magnitude.
The environements are way more interactive now - you'll operate cranes, ventilation systems, security systems, servitors, CATs, Servo-skulls, ships guns, torpedos, etc etc.
Which, if true, is good because there was pretty no interactivity in the non-EE version of the campaign levels. Just some consoles to click on, doors to either open, close, block, unblock or smash down. And I think I had to shoot a container as part of a mission objective once. And I guess there were plenty of explosive thingies like barrels and pipes and stuff in most rooms too. So I hope what he says is true. Maybe all this stuff has been saved and spent on the post-campaign "Special Missions" which I have not yet gotten to.
So let me just say this about Space Hulk: Deathwing. It is, by far, the most gorgeous and "authentic-looking" 40k game I've ever played. Better than Space Marine. Better than any DoW I've played. Every visual, from every angle is lavished with detail that screams 40k, and I think the game is worth playing as a 40k fan just to walk through all that for a while and drink it in. It's 40k done
right and so high res you can read the fucking scribbles of almost every banner and printed thing you come across, that's how detailed it is. Door frames, something so mundane in fantasy or sci-fi most don't think about them, are works of art rich in gothic detail.
So is it better? There's certainly more to do with your time. But the emphasis of this is clearly MP and the Special Missions. As a SP experience its legs are only so long, and let me tell you, the AI is not something you can lean on for something beyond casual play.
Get it because you want to experience being a terminator and stomping through the dankest 40k environments yet crafted by man. On that front it will not disappoint.