As seats, the C seats are more comfortable, but they don't go flat. You do get a lot more space in F. IME the food is about the same, and I'm not so sure the extra service is worth it. I don't really need to be fawned over when I'm turning on the light to eat my meal.

If you've got the miles to spare, get the upgrade. You'll at least get a data point on UA's F product.