Dont ever choose a job in flying for the equipment. Look at which is the more stable company, salary, benefits, perks etc. The 767 is a good aircraft and a 'HEAVY' so you would easily get another job due to heavy experience.
The 767 will be around for years in the cargo arena with a new upgraded cockpit for the 767, it looks like a G550.
Cargo operators are doing this as it allows the aircraft to be utilized in a more effective way. There are plenty of 767's in use outside of the US as well.
From purely an aircraft point of view go for the 787 but from a common sense point of view look at the stability and opportunities within both companies and where you want to go in life.
Dont let anybody tell you as a 767 pilot with say 4000 hours on type you are unemployable because it is an old design, thats just utter BS!!