Not sure which ereader you want to purchase? The following links give you pros and cons of different versions.
eReader Leader gives a few comparisons between major brands and their different versions as well as a list of things to look at when you are considering what ereader to buy.
Wikipedia also has an article that compares ereaders, including some name brands of ereaders that you may not be quite as familiar.
WikiHow – How to Choose an eBook Reader gives a step-by-step guide for what to look for when choosing an ereader.
TechRepublic – Comparing the Top Four eReader Apps gives a comparison of four free ereading apps for smartphones.
SummitSeries for Families has an article that not only compares apps for use on smartphones, but for tablets and computers, too!