I find that in many songs, either a G or a G7 will work well--they're essentially interchangeable. On the other hand, there are songs where one seems clearly preferable to the other. When I was starting out, it seemed that in folk music it was almost always G7, whereas in bluegrass it was always G. I question whether it's a matter of U.K. compared to U.S. so much as a combination of the type of music (blues, jazz, folk, bluegrass, etc.), personal taste, and what you're accustomed to.