Converting 24-29.97 fps or vice versa? Smoothest conversion.

A friend of mine just mixed 29.97 fps with 24 fps in a FCPX timeline.. I saw it on YouTube. He was asking an opinion. It looks like he's mixing the frame rates, and the 24 is looking particularly jerky.

If I'm to advise him, which gives the smoother playback: converting from 24 fps to 29.97 fps, or vice-versa?

I can then advise him to convert the one or the other so the whole timeline is exactly the same frame rate for export out to master.

I would do this by having him make a new project of the frame rate (that would make the optimum smoothness), drop in all the clips, replace the clips that are already at the correct frame rate (with slugs), then output the clips that were the no-longer-wanted frame rate ... to get a good clean conversion. THEN bring those clips back, now as a single QT contiguous movie, and drop that into the original timeline, now all the clips the same frame rate. He can use the slugs in the converted QT to easily position the converted clips over their original (wrong frame-rate) clips and replace them with the new correct frame rate conversions.

Maybe that's over elaborate.

But I do remember every time I've tried to combine different frame rates on a timeline, I get spotty, jerk results. And whenever I've converted the errant clips (wrong frame rate) to match the other correct frame rate clips (as a separate output) and then reintroduced the now converted clips to the original timeline I got smoother results. Albeit, I did the conversion with Compressor, which seemed to give me a smoother frame rate conversion. I don't think my friend owns Compressor, so if my impressions of a better converting job done by Compressor are incorrect ... it would be a nice thing to know.

Ben