Well, it depends on how much of the basic sciences knowledge you have. If you are from a Carribean medical school or a foreign doctor who haven't studied in a while chances are your basic sciences are pretty weak. So if your basic sciences are weak, you should not start with First Aid unless you have a tutor or supplement that with major book source like Kaplan. And after you get a strong base from Kaplan then you can start with First Aid as your only source. Now, if you only have a few months to take Step 1 and have a weak base go with the tutor who is experienced with First Aid. I understand that tutors can be expensive, but it's worth your money! And it will save your study time. Also just by having a tutor guiding you, you will feel much more comfortable and confident. If you are interested, check out my tutoring session at www.USMLEStepTutor.com or contact me
I tutored many students for the Steps, and the only source I use is First Aid and USMLE World. All of them passed with a good score. So give it a shot and see if you like my teachings. Good luck everyone!