“It’s about 50i I write not really – because it shoots i p, and then it’s encoded.”
I don’t quite get what you mean by that but I would suggest testing both modes and checking the results on a proper broadcast monitor- i.e. not just in FCPX.
Based on the specs you wrote I would recommend the 50p because it has the highest bitrate. De-interlacing footage in post generally produces objectionable motion artifacts and wastes a generation of quality.
Noah
FCPWORKS – FCPX Workflow
Call Box Training