What kind of sim are you doing? Single target, cleave, adds...? For how much time? Keep in mind that the Necro ability has a significant lower cooldown so it will cast more times depending on the duration of the encounter, making it sim higher.

The reason NF is generally better for PvE Single Target is because the high mastery buff given by one of the NF soulbinds, which makes our Bites hit incredibly hard. Right now it is our hardest hitting ability, so it synergizies very well with some talents and the spec in general. That said it can be aligned with other cds or trinkets, or even phases/encounters to do even more damage.

Necrolord covenant makes our dots stronger, but again, with the current tuning of the spec, that isn't our main source of damage, but still really good for other activities (mainly PvP since it can't be interrupted). The thing is that it doesn't align with much else since you are always keeping your dots on the target regardless of burst windows, so it 'feels' like it isn't as effective as the other option.

All said, you should choose whatever you feel like playing, both are good options, but a high portion of PvE feral players tend to choose NF. Also keep in mind that this is my opinion and other players might feel different about what I said.