I am not looking to extract still frames but rather want better quality (less noise) for the video.
If you want to extract stills - just not used ND filters at all and go for faster shutter speed - all will be crystal clear and sharp without any motion blur at all
Tody I recorded some test with 60 FPS and 30 FPS
30 FPs bitrate 112565 kbps
60 FPS 135494 kbps
So double the frame rate but the bitrate is not doubled - that makes me thing that 30 FPS supposed to be a bit better quality that 60 FPS - however visually I hardly see much difference
Gang, video compression is a rather complex system where bitrate isn't going to scale with framerate, while quality can be just as good.
Data "quantity", and therefore bitrate, depends greatly on the difference between frames. Simplified, a complete frame is captured and compressed using similar compression methods to a jpg (an I-frame), and then for a number of frames the
difference between the current frame and the previous frame is calculated, and that data is compressed. When there is very little change frame to frame, the difference "image" is highly compressible (think of it as a lot of runs of zeroed pixels, again just an analogy).
Periodically another I-frame is sent, starting the "baseline" for difference calculations over again. This is a configurable part of the image processing pipeline. I-frames are data intensive, so you want to send them as infrequently as necessary, but then has subsequent frames depart from the last I-frame data size per frame increases as well. It's a balancing act.
There are more complexities, like B-frames that are backward referencing, and other technical tricks, but this makes the salient point to understand why 60fps can be compressed with the same quality as 30fps without increasing the bitrate much... Adjacent frames have even
less difference from each other at higher frame rates. Two frames that are nearly identical can result in the latter frame compressing to almost nothing without compromising quality -- think of a symbol that says repeat previous frame (not how it actually is encoded, but makes the point).
The same principal applies to 24 vs. 60 fps. Yes, we'd expect a higher bitrate, but not by much.