I believe the difference between the two methods is previewing comes before you Photoread and Postviewing comes after you Photoread. Previewing is the process whereby you get an idea as to the books structure by looking at index, contents, headers, etc...
Pete, mentioed he preferred Postviewing ... probably because once you have Photoread the material, viewing of material should seem familiar and comfortable as you have exposed it to your subconscious mind.
In the end, choose what is comfortable to you ... the PR steps are guidlines meant to be modified to suit your needs in helping to master the process.
Good luck and stick with it!