The two proposals using the 767 and 797 models can be measured side by side in a theoretical look.
The 797 would beat the 767 in every category listed except maybe the long range distance which is yet to be determined in the theory model. The 797 will be lighter, faster and somewhat roomier than the 767-300ER. It will incorporate the latest Boeing 737 777, and 787 innovations. The to be determine column (TBD) of the 797 is an unknown quantity but Boeing can easily achieve an improvement over the 767 in every category. The only compelling reason for buying the 767 at this time is a bigger deal is in the works. Buying a 767 now is an order for the 797 launch announcement later. The expected life of a new 767-300ER would be in the ten year range where the 797 would replace it, unit by unit at a preferred customer price. Not only would a new 767 order become very cheap it would lock in a low ball 797 price later.
All this talk about a 767 order is really talk about acquiring a 797 fleet within the next ten years and United Airlines would be a prime launch customer for the 797 signaled by all this 767 talk.