This is a screenshot of the Impress Slideshow in action. Impress has more different slide transitions and effects. But it lacks the ability to create several slideshows easily. And within those slideshows, Impress does not have the ability to create fullscreen background for slides, return to first slide, a broad choice of fonts, and point-by-point reveal of a slides content as in the HTML5 Presentations plugin.
But otherwise the two plugins are fairly equivalent. However given a slightly faster response time for HTML5 Presentations we have decided to go with HTML5 Presentations over Impress. But be sure to consider both because its really a case of choosing your own favorite.