Showit Web added a missing piece that we couldn't have with SlideShow Pro, music. With Showit Web, we are able to add music and sync the picture transitions to the beat, or any sort of timing. We can use the license free music included, or we can add our own and time-sync it ourselves. Probably the other biggest selling point for us was the ease of use. With Showit Web, we are able to produce an awesome show in a very small amount of time. Attached is an example of what I did in 5 minutes with basement remodeling photographs that I have.
(please keep the laughing to a minimum these photos are unedited)