What's the opinion on country-specific guides versus the continental guide?

I have the new RG to South America, but the focus of my trip has now changed to concentrate on Bolivia and Peru. I note RG publishes guides for both countries - will I honestly be getting that much more in-depth information than what is already covered in the SA guide? I'm a huge fan of RG and have lots of other titles and am not averse to buying more but would hate to discover there are only a few extras that I could live without.