Get heavy Boeing time if offered. You already have 737 experience even if it is not NG - it's close enough. This also depends on your long term plans. With 767 experience, there is a higher likelihood the Middle East carriers would put you into a 787 or 777 if that was of interest for you (not for everybody...).
I'd go for the 767.
It is a classic aircraft and you will likely gain new experiences from flying it. Transition into a bigger Boeing aircraft down the line would probably be easier too.