More attachment points on the duplex lite vs. the ultra.
I have the ultra and wish I had the lite for the above reason. There isn't much weight savings at all. Both are outstanding frames and you can't go wrong with either. Just figure out what bag or bags you want to use and how much stuff you want to add to see if you want or need the extra attachment points.
One nice benefit of the extra attachment loops, is when you run something on the load sling, you can lash it directly to the attachment points that arent being used by your bag.