I have the same scenario leaving 3 weeks from today doing the 777 (DFW-DXB) for ~16 hours and have done a lot of research on exactly what economy seat to pick. I went the route skywardhunter suggested and picked a twin in row 48. I went a little further back in the twin rows because some folks said if you set in that first row where it goes down to twin (46) there is some bumping as folks come by and adjust as it goes with the extra seat. I don't do window seats so I picked 48B specifically both routes. My later A380 flight to KUL I went twin also on the upper deck in around row 30, stuck with economy all the way around.