I don't think that 30fps in inherently choppy on a 60Hz monitor, it's just that a game or render that is running at 30fps will have frames with a very high frame latency and frames with a short latency so some of the frames would be considered 60fps while others 15fps.
An example is that dvds do not appear choppy on a modern tv screen even though depending on the region, dvds run at 24 or 25 fps and the tv is 60Hz (usually). This is because that videos have an even frame latency so every frame is being played just fast enough to be perceived as movement without any that would make it seem like a slideshow.
EDIT: Back on subject no 60fps wouldn't be choppy on a 120Hz monitor any more than on a 60Hz monitor