it's not that you "should" use one over the other, but evaluate both to see which works better for you. people are excited about G+ because it's new, it's different (yes, lists and circles are basically the same, but their importance to the platform is different), and it's got google behind it, which seems to make everyone get excited.

i personally like it, and enjoy the list/circle-centric nature of it, but if none of my friends are active on it, there's no reason why I "should" use it over FB.