Just my opinion, but I think First Aid is much more comprehensive than Step up or Secrets.
Secrets is kind of like the First Aid Cases book. It's easy to read a couple cases here and there and it helps to reinforce the info, but it's not a comprehensive reference like First Aid. I read a lot of Secrets last summer, and while it does hit a lot of the info in First Aid, it's not really set up to just sit down and review a specific subject, eg. diuretics. The good part about secrets is that it is an easy read and it puts a lot of the facts into a clinical context which might help with recalling and applying the info.
One of the strengths of FA is that they have taken a lot of the really good diagrams from some of the most highly recommended books in each subject ie Katzung for Pharm, Levinson for Micro, and put them in one spot. I flipped through Step up, and while it looked OK, basically very similar to FA, the lack of these high yield diagrams was one thing I noticed.
