I wouldn't READ it, but use it for reference. If this is for a class, I'd get the big one b/c it has all the images, tables and the CD. It's a keeper in my mind b/c it doesn't matter what field you go into, this will be a good thing to have. I relied on it heavily during class but I think I actually only really read one chapter. If you're talking purely board study, the only thing I would get would be Robbins Review of Pathology (the highly-rated "Robbins Question Book") and use RR path or BRS as your base.
Pocket Robbins is two lies for the price of one: you won't keep it in your pocket, and it's not as good as the big one.
I have the 6th edition and used it all the way through 2nd year. I thought it was very useful and read it during each organ system. I also glanced through it during board studying and found it refreshed things throughout the year. Both the 6th and 7th will be useful, it just depends how much you wanna spend.