Elizabeth, but I like both! Elizabeth is probably more common across all age groups. I know Elizabeths ranging from a 2 year old to Her Maj, who is obviously in her 90s.
Isabelle / Isabel / Isobel / Isabella there have been a lot of over the past 10 years or so, whereas it wasn't quite so common in my age group (80s baby). I knew one at university and always likes it, but she was the only one I knew. Wonder if it used to be more upper class? She certainly was very upper middle class. It's mainstream now I think and doesn't really have a class imo.