In the past 30 horus I have completely read step up and read the anatomy, biochemistry, and physiology portion of first aid. By tomorrow night I should be fully able to answer your question. Here is what I have learned so far:
Embryology: Step up is a TON better than first Aid
Physiology: Both covered the same thing
Histology: Step up is better than First Aid
Biochemistry: First Aid has a lot more usefull stuff.
Anatomy: I like both about equal here, with the edge going to step up barely.
Microbiology: (read the FA part a month ago) they have a layout for this that is about the same. FA has better pictures of the bugs.
Pharmacology: Havn't read FA yet, but Step up includes every drug known to man. I liked reading through it because by the end I was easily able to recognize classes of drugs by their roots even though I hadn't seen that drug before. Also helped beat the side effects of the classes of drugs into my head. (ACE inhibitors damage the kidney)
Overall Layout:
Step up is made so that you can understand the material a ton easier. I like the quick hits, in part because I took a Kaplan Practice test and a lot of the questions were strait from the quick hits.
First Aid is good if you already know most everything, because it is hard to read through the stuff if you don't know it. Great for the nuemonics (horrible spelling) and stuff like that.
I personally would recommend getting both books. Tomorrow I might finish my analysis of the two.