Oatmeal is the preferred carb since it is low glycemic however since the gastric emptying rate is slower it's not a fast energy source like high glycemic juices and sports drink. In the long run, low glycemic foods/drinks are healthier. If you're the type that gets hypoglycemic, you probably need to start your hike a couple of hours after the oatmeal breakfast whereas a shorter hiking time is only needed for simple carbs. Having said that, a study which involved a group of endurance cyclists showed that there was no significant difference in performance regardless of the glycemic index of foods when eaten 1 hour prior to exercise. I guess in your case it depends on the person.