You want to record in 4K.
For all movies, you want to upscale before interpolation. It is much slower, but the results are worth the extra time it takes.
I was thinking if Topaz is better at creating additional frames for slow motion or upscaling existing frames from a HD footage. I would have thought that the latter would be easier.
Definitely start with the best quality video you can and then apply the frame interpolation to that.
Also just be aware when filming your video of any light sources as they can strobe when slowed down if they are out of sync with the shutter of your camera.
Thank you guys for all the advice. I will start shooting in 4k for my iPhone and mirrorless camera and slow them down via post processing with Video AI.