for me it is not a big difference, most people say it is a bit more important on c but i believe with Josef tool the PF has a sligthly bigger impact on rating. Hopefully i confused you even more with representing all possible answer in one sentence *gg
But i would judge the defensiv assigment this way:
1. put the guy's with the better OD on the PF position defensiv
2. if this is close together, use the guy(s) with the better ID on the stronger big men of the opponent
3. put the best rebounder on C