user-1: The camera here is a Samsung HMX-Q10P (P likely stands for PAL: the N model would have 60fps instead).
The videos I upload usually are 25fps either way, but strictly speaking 1080/50i will never give me the odd lines when dropping every second frame, so I rely on the deinterlacing algorithm of my video editor (Shotcut in this case which may use ffmpeg or other stuff internally) to deliver reasonable content.
I've made a few attempts at fast moves of the camera shooting 1080/50i and I have not been able to see noticeable staircasing when creating 25fps output from that by reexporting from the video editor.  However, I am not sure how much quality deterioration the interpolation of the odd scanlines will cause.
So assuming the desire for best results, would I be better off shooting in 1080/50i or in 720/50p for videos without global movement (fixed mount, no zoom while shooting) and moderate local movement (close-captured single musician), and how would those considerations change given different material?

user5747: If you're mainly concerned about the staircasing, consider where the video will be watched. If interlaced video is shown on a display made for interlaced video, it appears ok. If you play it on a progressive display (such as a computer monitor) the staircasing is easily noticeable even for little horizontal movements (of the camera or of the subjects). For details see https://cardinalpeak.com/blog/interlaced-video-and-computer-monitors/
Best is if you never have to deal with interlaced video if possible.

-1: <p>The camera here is a Samsung HMX-Q10P (P likely stands for PAL: the N model would have 60fps instead).</p>

<p>The videos I upload usually are 25fps either way, but strictly speaking 1080/50i will never give me the odd lines when dropping every second frame, so I rely on the deinterlacing algorithm of my video editor (Shotcut in this case which may use ffmpeg or other stuff internally) to deliver reasonable content.</p>

<p>I've made a few attempts at fast moves of the camera shooting 1080/50i and I have not been able to see noticeable staircasing when creating 25fps output from that by reexporting from the video editor.  However, I am not sure how much quality deterioration the interpolation of the odd scanlines will cause.</p>

<p>So assuming the desire for best results, would I be better off shooting in 1080/50i or in 720/50p for videos without global movement (fixed mount, no zoom while shooting) and moderate local movement (close-captured single musician), and how would those considerations change given different material?</p>
 5747: <p>If you're mainly concerned about the staircasing, consider where the video will be watched. If interlaced video is shown on a display made for interlaced video, it appears ok. If you play it on a progressive display (such as a computer monitor) the staircasing is easily noticeable even for little horizontal movements (of the camera or of the subjects). For details see <a href="https://cardinalpeak.com/blog/interlaced-video-and-computer-monitors/" rel="nofollow noreferrer">https://cardinalpeak.com/blog/interlaced-video-and-computer-monitors/</a></p>

<p>Best is if you never have to deal with interlaced video if possible.</p>
