Hi Susan, FiatLux is right, both are useful for practice!
You may notice that one helps you more than the other, so just experiment and find out. I am a huge Snellen chart fan, and have two pages on my blog you might be interested in. The first page is more general, how to use it and how to get measurements from it:http://sorrisi.wordpress.com/snellen-chart/
The second pages details how to improve eyesight with it:http://sorrisi.wordpress.com/2011/12/06 ... ce-part-1/
The chart has been immensely hepful in my own vision improvement.
see my vision diary blog: http://sorrisi.wordpress.com
After 8 months: 20/50
now, 5/10, 10/20, still improving and loving it