I don't have the 6th edition but i have used both 4th and 5th and there was nothing significantly different between the two. All the same just some more color and better organization if anything. I would jsut stick to the 5th edition. As for costanzo, theres quite a few info in that book. But coupled with qbanks and brs phsyio you really don't need the costanzo book for step 1 review. good luck