United will likely receive the first aircraft, but Delta should receive their first not too long after United. Delta needs both versions to replace their 767-300ER and 767-400ERs as the 400ERs will be coming up on 30 years by the end of next decade. Delta will probably split between the smaller and larger version of the aircraft. The rumoured 797-8X will trade size for range and will have a range of 4,200 nm, a bit further than their 75S fleet. This a plane for JFK/DTW-Europe while the 7X, smaller and longer range at 5,000 nm, will do ATL-Europe/Deep South America. I expect Delta will order 60 of these and may go for a small A321neo LR fleet for flights to Europe that are just too small for the 797. Deep South America is probably switching over to mostly A330ceos and Pacific flying A330neo/77L/A350.