I used OpenBox for quite a while and liked it much more than Fluxbox. I prefer minimalist WMs and Openbox suited me perfectly.
In Openbox, you don't have any toolbars or menus, you use what you want without getting rid of the default that comes with things like Fluxbox. I also felt the configuration (key binding, menus, etc.) to be much less restrictive, and, contrary to what many will say, much easier to navigate and configure. obconf really simplifies the configuration of OB, I don't think there is a similar tool for Flux.
They are both tiny, nobody can tell you which you like more...so go and download/try all of them.
Flux has a bit more eye-candy than OB, but loads noticeably slower, in a nutshell.