Next to First Aid, RR Path is the single most important book you need for Step 1. I own the second edition. My friend has the 3rd edition. I flipped through the pages of her book and I didn't notice any major differences. The updates were very minor (such as correction of the errors in the 2nd edition, but you can download the errata and corrections from the internet). If you already have the 2nd edition you should be fine. If you want extra reassurance, go to a bookstore and browse through the 3rd edition.
I had this same questions months ago. I ended up buying the 3rd ed. I'll agree that it is easier to read. There are a few more pictures and charts. Overall, the layout and organization is better. However, if you've been using the 2nd ed and know it at a comfortable level, it may be better to just stick with it. As mentioned above, flip through the 3rd ed at a bookstore or library
They pretty much have the same material so it's prob whether or not you think you'll use it enough to make it worth the money. The main benefit is it has more pictures and more margin notes. The format makes it seem less dense as well.