A unicycle does not have "about the same speed as the small wheel folding bikes". There's no glide to a unicycle, and no gearing selection. Those small bikes ride quite well, you should try one.

The major difference in a small-wheeled bike are stability and and ability to handle large bumps (like curbs or pot holes). Short stops might also be a bit more likely to send you over the bars, and the folding mechanism might not be up to X-Games type activities.