I played both on Xbox 360, DA:O Ultimate Edition and Dark Souls in the same year a couple of years ago. While DA:O was longer, more open, quite entertaining, great story and characters, diverse (world, enemies, decisions), with more ways to define your own character and the way to play it, and compared to Dark Souls, "conversations and voices"; Dark Souls was shorter, smaller, linear, repetitive, had absolutly no story and a poor array of sounds, punishing and not that great... Actually, I thought it was a little overrated...
Graphically, Dark Souls is not that detailed than DA:O (but it's close) and it's much darker than DA:O (some levels are pitch black), the levels are very linear and not much too see (they are big, but where you can walk is really 'claustrophobic', with invisible walls everywhere), and where it gets more open, there's not much to do than walk, walk, and walk until you find something to kill, and believe me, you'll do A LOT of backtracking just to get something. If you re-visit an area, you'll have to kill the same enemies in the same places again. Once you finish the game, you can re-start the game with the same character and levels, but it gets more difficult the next time (enemies are harder to kill -have more life- and do more damage). Although, the enemies get tougher, they are not more evolved, you kill them the same way as before. If you get killed, you return to the last place where you saved the game (a place with a bonfire where you can rest and recover HP), and are punished in some way like losing something, but I can't remember what was it... (:P)
The game is hard, but not really-really that 'impossible' hard that some say, you only have to think how to hit an enemy, use defense, shield, or avoid attacks a lot, and have patience. In others words, how to fight properly. People who only use the attack button have problems with the game.
I finished the game three times in a row, found everything -including all the secrets-, did and killed everything, and my character was a little overpowered by the end. I stopped playing it when I was close to end it for a fourth time, but that was just because I was really bored of the game and had better games to play... (:P)
If you plan to get both, that's great, but if you can get only one of those, I highly recommend Dragon Age: Origins over Dark Souls. The Ultimate Edition has all the DLC and extras, which are extra items, an extra character to meet during the main game, some backstory in others and extra post events to the main game. Get it if you can, because the Ultimate Edition on PC is not available in all regions (like mine... --_--), and because the DLCs for Leliana and Morrigan were great ^_^
Post edited December 23, 2013 by Azrael360