As the name implies, the emphasis at SlideShare is on sharing. To be fair, Zoho has recently introduced some social sharing features, too, but Zoho is newer to the game, and it remains to be seen how lively their site will be. For the present, SlideShare remains the "place to share and discover presentations and slideshows." As at Zoho Show, you can upload a presentations for display over the web. You can also convert presentations to embeddable Flash objects. SlideShare will accept PowerPoint, Open Office, Keynote, and .pdf presentations.
A major part of the point at SlideShare is to make your work available in what you can think of as a presentation marketplace. It's YouTube for slide shows.
When you upload a file, you'll be able to specify whether you want to allow viewers to download the original file or not. After upload, you'll be able to link to your presentation on SlideShare or embed it on your own web pages. Here's a link to my show's page at Slideshare. You will notice that SlideShare is making it easy for others to make links to your show. In addition, SlideShare has a host of social features: You can tag your shows, search for shows, add them to your favorites, subscribe to particular users, create or join a group, and more.
Here's an embedded version of my show as converted by SlideShare:
The quality is roughly comparable to Zoho, but the viewer is slightly less sophisticated. In particular, I miss the the option to toggle to a full screen version. Still, if sharing is a priority, then SlideShare is a good way to go.
A distinct advantage over Zoho is that the embedding for SlideShare seems to be unproblematic for course Blackboards. Of course, as with Zoho, you can also simply make an external link in your Blackboard that will "display media file within the page."