Based on that nutritional breakdown in jno's link, I'd still be inclined towards walnuts.
Pecans lead the way in monounsaturated fats, walnuts are the leader when it comes to polyunsaturated fats.
Pecans are higher in fibre and lower in sodium but walnuts have more calcium. A lot of people simply don't get enough calcium in their diet, and certainly not an absorbable form of calcium.
It's also worth knowing that the fatty acids you get from oily fish are far better than those you get from nuts and seeds. However if you don't eat fish, nuts make a sensible alternative source.