They are very different.
First smart power is only applied when data rate is selected to 6.8Mbps. It does not work in other data rates.
Second forcePower just tells transceiver to use whatever user set in the tx_power register, other than the recommended values. So even when smart power is set, force power value still applies.
Lastly, forcePower only exists in driver code. the transceiver chip only stores smart power flag in register. So in low level smartPower uses whatever value set into the tx_power register. Forcepower can be ignored if you have direct access to chip registers bypassing the software driver.