Several differences. Real Scenery is a set of 3rd party payware that covers a couple of specific areas. The terrain textures are based on actual satellite or aerial photos of the area, so as long as you're high enough that the resolution isn't an issue, it will look just like the real thing. The descriptions don't say anything about 3D structures, so I can't comment on what it will look like at low altitude.
The Global Scenery is indeed global. (See above for exact coverage.) You can count on having every little island in the Caribbean, south Pacific, etc.
The global scenery is all generic, however. It uses land use data and highway databases to figure out what textures to use and where to put streets and what kind of buildings. The textures and buildings all come from a generic library (an extensive one!) and are not the actual ones. From any kind of distance any major city will look quite realistic because the land use data results in the right kind of buildings, but look up close and the buildings won't be the same as the real ones there.
The strong point about the global scenery (other than that it's truly global) is the buildings. Many photo scenery packages look great at 10,000 feet, but get low and they're worthless, because all you see is a flat surface painted with colored blobs. Get low in the global scenery and you'll be dodging detailed buildings. To understand what I'm talking about, see the screenshots at the Global scenery web page. Understand that the level of realism will vary. The quality of highway and land use data varies for different parts of the world, and I've seen some bogus land use just in the US.