The key difference for me was the way they create the names of the links in the popup. In Chrome you need to enter more or less random letters, while in vimperator you can write the link name. The difference in usability was HUGE for me and made me switch to FF for a brief moment from using Chrome since its release day.
I have trid vimium on chrome, vimperator is more configurable, you can change the theme, the suggest, hide the navigation bar...