BT.709 for PAL SD

This is a question regarding color transfer characteristics in encoding to FFV1.

Research for vrecord saw BT709 implemented for all FFV1 capture:
[https://github.com/amiaopensource/vrecord/blob/50e9dbadf9f05f640f0ab7cf95c7e428c62adb69/vrecord#L881-L895](https://github.com/amiaopensource/vrecord/blob/50e9dbadf9f05f640f0ab7cf95c7e428c62adb69/vrecord#L881-L895), including PAL SD. This seems to show that development is in favor of using the BT.709 profile for color transfer characteristics mapping (ffmpeg “color_trc”) of PAL SD video, instead of the “gamma28” profile (BT.470 B/G), even for SD. Normally I would’ve expected that color transfer would also follow BT.470 for PAL SD.

Dave Rice comments :
"[these comments] came from this section of vrecord which sets color metadata for SD captures in vrecord, conditionally, whether they are NTSC or PAL: [https://github.com/amiaopensource/vrecord/blob/main/vrecord#L1873-L1894](https://github.com/amiaopensource/vrecord/blob/main/vrecord#L1873-L1894)."

...and further :

"In the QuickTime specs at [https://developer.apple.com/documentation/quicktime-file-format/color_parameter_atom](https://developer.apple.com/documentation/quicktime-file-format/color_parameter_atom), in the transfer index the transfer function value 5 is reserved. But now transfer function = 5 is regarded at bt 470 b/g. Also at the time, we were trying to get vrecord to emulate Final Cut Pro and I believe it used the same transfer function = 1 value which used the same values as bt709."

For encoding from PAL SD, which is the better choice? BT.709 or BT.470 B/G ?