Always NDF with 23.98p source.
Downconverting a 23.98p tapes requires a decision on timecode (TC).
TC from such HD tape can not be regenerated on SD tape formats because one runs at roughly 24fps and the other at 30fps. In this case TC will get converted and you will have a choice of going either DF or NDF. Ask for NDF and “A” frame at 01:00:00:00 or you may be in for many sleepless nights.
An exception to this would be the case of downconverting a finished master. The SD downconverted master can be created with either DF or NDF timecode.
When shooting 1080 59.94i the TC on the downconversions should always be regenerated from the camera master.
Downconverting from 720 59.94p is similar to 1080 59.94i. The original camera TC should be regenerated on the SD tape.
For additional discussion click here.