Engines,
Agreed. The OEW difference is quite big, though, compared with Rafale (1360 lb, I think), even accounting for the fact that JSF is bigger overall. Wonder why?
Also, I remember being told by the program folks that the big reason for the gun switch was logistics costs, 27 mm being a completely new calibre for the US. And, at the time (2003) they were saying that they had lots of weight margin to play with...