This post is my initial review of the two services. It features this post and a Google Document that is shared below. Feel free to add your comments below and modify the google document with new insights, edits and other stuff that can help us all evaluate these two services.
Embedding
Below are the options available to anyone wishing to embed a presentation on a blog, wiki etc.
Slideshare (embed options)
Option 1: "Embed"
Twitter Personal Branding
View more presentations from John Moore.
Option 2: "Embed Without related presentations"
Twitter Personal Branding
View more presentations from John Moore.
Brainshark (embed options)
Option 1: Size: 528x439
Option 2: Size: 440x366
Option 3: Size: 422x351
Comparison Chart
Below is a screenshot of a publicly available and editable google document that you can access here.

The verdict? I'd give Slideshare the nod but I'm sure there are lots of really great people at Brainshark who are busy building a great service.
Feel free to leave your comments below regarding your experiences with either of these services, share links to example presentations (they can be yours too, just make sure it has something to do with this post). You can also contribute to the google document here.