Honestly I would argue it doesn't matter in either case.

If it is about possession, there just isn't enough of a negative effect from allowing someone to look at that stuff for another month or so to justify serious infringement on rights without a conviction. The abuse has happened, and a single individual looking at it some more won't affect things much. And after an actual conviction they'll just be in prison, or after release you would have a justification to monitor at least their own internet traffic.

If it's about production, the internet traffic isn't likely to be the problem. Someone sexually abusing children isn't likely to stop just because they can't put it on the internet anymore. At that point you'd rather need to keep them away from children in the first place.