I bought and used First Aid, but I did check out Step Up from our library and was really impressed by it. I loved the sytemic approach, but it didn't really tell me anything that First Aid hadn't already covered, so I didn't use it. Plus, First Aid for Step 1 has a great reputation among students, so I stuck with that, and it seemed to work out for me
edit:
Now that I just read Jalby's post.... ok, yeah there are some new things in Step Up, but to me they didn't seem like anything I couldn't get from any of my other review sources, so i didn't buy it.