Having had both for a bit over day now, and having bit in a good few hours on each, go with Dark Souls. Dark Souls hardly lives up to the reviews that are praising it as perfection, and it's really not a "can't put it down" type of thing, but it's still rather good and can be very rewarding when you do things right and get through an area alive for once. Rage... Is hardly horrible or anything, but in the end it's about what you'd expect, a generic wasteland shooter. It's alright, but certainly shouldn't be on any wish lists, nothing there you haven't seen and done many times before.
Honestly, I'd say Dark Souls or neither. Dark Souls really depends on what you enjoy, and if you liked Demon's Souls it should be a definite yes, but Rage is hardly worth bothering with at all, just don't waste your time.