Elton John & Billy Joel?

I am going to their concert the week after next. I don't know a lot of their songs, I am just going because they are so big and who knows if I'll get a chance like that again. I also got a sweet deal on the tickets. I wanna get some cd's to get in the right mood before the concert. It's always more fun when you actually know some songs and you can sing along. So i decided to get "Elton John - Greatest Hits 1970 - 2002" but I am not sure what cd i should get from Billy Joel. I don't think I wanna get more than one and there are just so many. Please help!