Prezi
I have looked at Prezi before but did not use it. Since I joined up as a student I was able to get the free educational account (If you have an email account with a school domain you are eligible for this account). This account enables users to have the option of making private presentations, great for those of us just starting! It is very easy to get started making Prezis by using the ready made layouts (many layouts are available and some people make their created layouts freely available). Make sure to look at some "how to" presentations for tips, especially on how to make the presentations flow well and not be nauseating. I really enjoy how it is so easy to import PowerPoint slides into the presentation. Also, Prezi deconstructs the slides so that the pieces can be fine tuned. Another thing that Prezi does very well is it's ability to embed media, such as YouTube.
Slideshare
Slideshare is very easy to start using. Just upload your PowerPoint presentations and off you go! A great way to make your presentations quickly and easily available. I also like the ability of Slideshare to be easily embedded into websites. However Slideshare defiantly falls behind Prezi as it is not dynamic since it cannot embed video or have the ability to incorporate PowerPoint animation schemes. It is a very straightforward medium that is great to start off with. I have a free account with Slideshare and would like the option to make my presentations private.
Both Prezi and Slideshare offer great examples of how to present and are full of great presentation ideas! If you are just starting to bring yourself online Slideshare would be the easiest to start with. However, you should still check out Prezi to see if you like it, as it offers the ability to create more dynamic presentations and enables you to be as simple or elaborate as you like. If you are interested in embedding videos, Prezi is the way to go.