I've only used USMLE world so far, and I think its great.
I'm almost done with that, and I will try Kaplan and USMLE Rx next.
My understanding is that Kaplan tests more on minutia (which may make it better for following along with class), and USMLE Rx goes hand-in-hand with first aid. Rx also has the most questions, I think, and is the cheapest.
I will let you know what I think, but honestly, I'd do as many questions as I could get my hands on. I learn better that way though. I'd fail boards miserably if I tried to study by just reading first aid.
And if you get a copy of FA early, I highly suggest getting a new one and starting from scratch when you start your board prep. Your old copy that you used for class will be cluttered up with bull****.
I used FA for a few mnemonics and stuff during classes, but I didn't look at it too terribly much.