As the operating temperature increases, the theoretical operating voltage for a fuel cell decreases and with it the maximum theoretical fuel efficiency.
On the other hand, increasing the operating temperature increases the rate of the electrochemical reaction and Thus increases the current which can be obtained at a given voltage.
The net effect for the MCFC is that the real operating voltage is higher than the operating voltage for the PAFC at the same current density.
The higher operating voltage of the MCFC means that more power is available at a higher fuel efficiency from a MCFC than from a PAFC of the same electrode area.
As size and cost scale roughly with electrode area, this suggests that a MCFC should be smaller and less expensive than a "comparable" PAFC.