When do you want to use a FlexPIFA over Flex PCB antenna?

Answer

In a cluttered propagating environment, such as a medical suite, office, etc. where there are many metal obstructions.
As a propagating signal encounters an obstruction, the antenna's polarization sense changes and will continue to change as each obstruction is encountered.
The FlexPIFA antenna mitigates this polarization sense issue by providing energy in both vertical and horizontal polarization; thereby, increasing the probability of picking up a useable signal.