From what I understand ATI is for content, Kaplan for test strategy. I have the ATI series and actually prefer Saunders for content. The ATI tests are ridiculously hard and make you feel super dumb. I've been told that Kaplan more accurately reflects the level of the test questions. Our school also made us get Medspub. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.