Had just under a pound each of skirt steak and flat iron steak. Marinated both for approximately 6 hours using Alton Brown's 5-star recipe (soy sauce, EVOO, onions, garlic, lime juice, red pepper flakes, cumin, brown sugar, run through a blender until uniform consistency). Grilled sliced peppers and onions to perfection on BGE griddle (Note to BGE: *Please make a griddle to fit XL Egg!*), seared steaks on cast iron grate, about 1 minute per side. Let rest, then sliced across grain and served with warm freshly baked flour tortillas. Amazing flavor!!
One important lesson I learned: from now on I'll be using only flat-iron steaks and saying adios to skirt steak. Both have great flavor, but the flat iron steaks are incomparably tender, not stringy like skirt steak.