Line to Line or Line to Neutral voltage to calculate 3 phase power
You can use both voltages to calculate power, but it yields to a different expression. If \$U_f\$ is the line to neutral voltage and \$i_L\$ the current in one the lines, then:

$$P=3U_fi_L\cos \phi$$

Now because line to line voltage \$U_L\$ is related to \$U_f\$ by \$U_L=\sqrt 3 U_f\$, then after substituting in the previous expression we have:

$$P=\sqrt 3U_Li_L\cos \phi$$