Why worry about how much is one or the other? Unless you are weighing a whole project...
2.8oz fabric is heavier than 1.1 oz fabric. Given the choice, go with the lighter one. Your kilometers may vary of course. Just sew the thing and when it's done weigh it on the scale of your choosing, ounces or grams or whatever. My underquilt came in at 1lb 11oz, in its stuff sack, despite using m50 for the shell. I was happy with it, however, and used it twice so far.