This is a great question; I consider Corey "lead designer" for FFG right now [regardless of his actual title, the cat pops up enough to warrant that level of recognition] and both DA & GOW are brilliant in their own right. So let's break it down right quick:
First, as was previously noted, Death Angle [DA] is abstract, except for the die rolling. You're really managing the formation and the horde, moreso than attepting to manage concrete elements. GoW is significantly more concrete, relying on the canon of the game to provide structure and framing, where Death Angel is a wildly restylized version of the brilliant Space Hulk .
Second, both games point weapons through card play. Resolution in GoW is more complex and thus more interactive (more on that in a moment) but broadly speaking, the underlying mechanics are similar. You are responsible for one COG, that COG has a hand that actively determines his options, and you make decisions from that. So instead of, say, six cards where you play 2 because you command two wings of Space Marines, you have one COG with a full hand which is also their health.
Third, both resolve through dice rolls, but here they start to vary wildly. You're accustomed to reading the single die in DA, and looking for the skull. In GoW, it's similar, but in many cases you're looking for a "6," or the Omen (a skull device inside a gear). This conveys special adjustments to the roller, whether enemy or ally. It's a system similar [or at least inspired by] Descent's complex die rolling mechanics, which I think are excellent, but stripped down to be more significantly straight forward.
Now, the big breaks.
DA uses a mission construction system that is fairly random, while GoWs is pre-set. Second, GoW has a significantly wider variety of enemies and thus requires more complex die rolling to resolve combat. Third, because GoW is a minis game, and not an abstract map, elements such as line of sight, cover mechanics and so on come into play. In DA, you can get your team shredded and be down to 4 Space Marines. Now, the Genestealers are in positions 5, 7 & 8. Good news. They keep moving (and growing in some cases) but they never attack. GoW doesn't have this "problem" because enemies are constantly moving after every turn on the map itself, so there's a significantly higher likelihood of getting jumped. And.
Just to add insult to injury.
Many cards read "if you don't have any X, discard and draw a new card." So the deck of pain is in fact out to get you.
Deep down, I love Gears as a franchise. It's machismo and melodrama, but it's got great big freakin' guns, great mechanics, and a neat twist in game 2. If you haven't done a "big box" board game before, Gears is a great place to start. The map tiles are good, and believe me when I tell you: play once, and you'll have the rules down cold. I would absolutely recommend it. If DA is good for you, please give Gears a shot. Also, try LOTR: The Card Game. Also excellent.