${\mathrm{P}}_4{\mathrm{O}}_6$ and ${\mathrm{P}}_4{\mathrm{O}}_{10}$ are different in :

No. of 

$\mathrm{P}-\mathrm{O}-\mathrm{P}=\frac{{\mathrm{P}}_4{\mathrm{O}}_6}{6}\frac{{\mathrm{P}}_4{\mathrm{O}}_{10}}{6}$

$P-\mathrm{O}-P$ bond length is different

$\mathrm{P}\to {\mathrm{sp}}^3 {\mathrm{sp}}^3$

Hence, the correct answer is $\mathrm{P}-\mathrm{O}-\mathrm{P}$ bond length.