Out of those two books, Kaplan is favored amongst others who are studying for the USMLE and those who have already taken it. However, what people really recommend as the best book to study, above all others, would have to be:
I have gone through both Kaplan and BRS patho both are very similar, it doesnt matter which u use. Most of the stuff is there in both books. Kaplan presentation of material is good. Goljan ofcourse rocks, a lot of extra material though, dont know if its all necessary.
It appears that you should never study for the USMLE from only one company/review set/series etc. Some people may find Kaplan wonderful in one area, while lacking in another, as each subject is done by a different author in a different style, and some might even be very similar but the thing is some subjects are taught better in a different way.
What I do is make a list of the topics I want to study, and then go to amazon.com and read reviews and find the standards that most people are using for each subject. Like for patho, everyone seems to Love Goljan, but for Pharma people love Katzung. Other subjects people go for BRS. I think this is a better way of going bout it