Both very interesting sites. Marvel was very in depth and I can see how much of a benefit it would be to all of us including my own kids with their homework and projects. The data base is so large and continues to grow. Plus the many links and other jump offs that can take you all over in search of content that you require.
Slideshare reminds me of Delicious which allows me to bookmark all of my favorites from work to personal sites. Right now I use my Delicious account a few times each week and continue to add to it as I come across any new sites that I may want to go back to at some point. Will I use Slideshare - only time will tell as I continue to expand my options yet stay in control.