Pentadactyl's ("5digits"?) build system is broken, and they haven't released a stable version recently. I publish my own personal builds from the unmodified source at this location[0], if you're interested.
Any highlights on how the two differ today? I seem to recall briefly using pentadactyl as vimperator broke something, perhaps "it's all text!". I don't understand how people can stand vimium - but that's probably because I've grown used to vimperator more than anything else.
I've never used Vimperator. I do know that Pentadactyl is far, far better than any solution I've ever used on Chrome because of how well integrated Firefox extensions can be and how limited Chrome extensions are (can't run on Chrome Store pages, can't run on builtin pages, can't run on file URLs, can't launch external programs, etc).
[0] https://www.rylee.me/~rylee/dactyl-builds/