You have not mentioned what PCB material you refer to.
If we assume a PCB with moderate permittivity and some loss, my estimate is:
FPC antenna has more field in air (outside substrate), so it has less dielectric loss => better radiation efficiency
PCB antenna has more field percentage in PCB substrate and is thus less sensitive to detuning by nearby materials.
Bandwidth is difficult to estimate, because it depends on losses (more loss => more bandwidth) and it depends what we compare (FPC antenna same physical size as PCB antenna? or both full size for the respective permittivity?)