having juddery video issues and I've noticed that iMovie changes all video to 30 fps, so if you film in 24fps obviously that causes a problem...has anyone else noticed this?

I have the same issue using iMovie. Now I switch to TunesKit AceMovi, you can change the project settings to 24,29.95,30,59.95,60fps with this editor.

My message does not answer your question, but I would like to know what the difference is between making recordings at 24 and 30.

I personally have not noticed, I generally when I watch movies on TV, I am amazed at how smoothly the picture goes on the screen.