I love Frank. So deadpan!
I prefer it on its own moreso than as a nickname. Francis and Frank don't really "belong" to each other to me, if that makes sense. I know Frank can be used as a nickname for Francis but, despite how close they are, it's always seemed a little forced to me.
In a GP sort of way I like Franck very much, too. No rhyme or reason to it but I enjoy something about that superfluous C thrown in there.