It seems to be a very subjective thing
This is 60fps:
- and unless your network connection is bad, plays back silky smooth. There's a gimbal pan at 1:48 and it seems pretty good to me. If there is motion in the shot (you're flying around or panning) higher frame rates will counter the natural blurring that comes with longer shutter and (in come cases) video codecs. For landscape type filming, I personally find the sharpness more pleasing to the eye.
We are used to a certain amount of motion blur in the footage we see though. It seems a lot of the criticism of the Hobbit (shot at double normal frame rate) was that the CGI effects looked too 'precise' and artificial - in fact a lot of CGI uses a heavy amount of motion blur to fool us into thinking it's natural and when that goes away, effects laden films do look odd.
I really don't see the value in shooting 4K and then downsizing. Theoretically you could gain a little bit of extra information, but that depends on a very smart resampling algorithm which most video editing software doesn't have. On the whole it just means you get massively larger files and have to spend hours reprocessing them. Resizing does have the side effect of setting the image sharpness, so that may give the impression of 'better quality', You can get the same result by filming in 1080 raw and then running a sharpening filter over it.
If you're shooting indoors, your main enemy is light - or the lack of it. The camera will naturally want to keep the shutter open longer to get better images, which makes things blurry. Turning the film to 60fps will force it to use a faster shutter rate, but you may find image quality drops off (cameras typically compensate by bumping up the ISO). Human eyes are very much better at compensating for dark scenes, so we often don't realise how little light there is indoors. Open every curtain, put on every light (even in daytime) and use video lights to compensate - a hand held video light can work very well.
120fps is only really useful for slo-mo effects - fun to play with and useful for specific shots, but otherwise not useful for day to day filming.
If your absolute need is quality, then always film in raw. It produces 'flatter' and slightly less sharp footage out of the camera, but that's perfect for post-processing. You must then bump up the contrast, saturation etc. in your editing software and add a sharpening filter. Tuning those well will produce a bright, punchy and crystal clear video that the in-camera modes just cannot match.