No current Macbook has hardware h.265 acceleration as far as I know, whereas any Macbook newer than 2013 does have hardware h.264 acceleration. So it'll be a much smoother experience editing h.264 files directly (if iMovie is anything like FCPX). To edit h.264 files in iMovie, you'd need to use some other software to transcode them to prores first.
The quality differences aren't so pronounced. I think the convenience of working with h.264 and not having to convert files to prores WAY outweighs the