Okay, this is the first time I've heard Openbox being touted as lighter than Fluxbox. Can't find a lot of statistics to back it up, but here's one:
http://debian.cante.net/stem/faq/index.
ow_manager
Openbox may be redesigned and take up less hard drive space and need less code, but memory requirements are higher. I've seen a few other sites that give similar statistics. I've run Openbox, Fluxbox and dwm on a 64 MB RAM system and done tests with Conky and top and lxtask. Every test came out with Openbox using about 1 MB more RAM on that system than Fluxbox. dwm used less (several MB less). In my comparisons I was running just Openbox, not LXDE (Openbox with other applications like a task bar).
I personally don't find the configuration files for Fluxbox any easier to work with than Openbox, although I have noticed it's easier to make a typo in the Openbox configuration files and have things not work. There are some good editors specifically for XML if anyone wants to give them a try instead of using a standard text editor with no XML support. Might help with editing. A programming editor can at least highlight XML syntax so you can see if you lost quotation marks or something. I like the obconf program, but obmenu requires Python. On slower, older computers, I tend to avoid installing programs written in interpreted languages when I can. They're almost always slower than compiled programs. If Python isn't installed already, it can take up a lot of room too. I personally find it easier to just edit the XML menu file in a programming editor.
There are a few things I like about Openbox that have made me end up using more often than Fluxbox of late. I like the ability to alt-tab through windows. Fluxbox can do it, but I find it doesn't do as smooth a job switching between windows and sometimes it brings up the wrong one. It's not an important feature, but I like the logoff dialog in Openbox as well. I find keystrokes when navigating the menus work smoother for me in Openbox than Fluxbox. I like having an empty background screen with Openbox (no other tools running) so if I run something like Xfireworks, it can use the entire screen display. I haven't made much use of them yet, but Openbox also provides pipe menus and I don't think you could provide similar functionality as easily in Fluxbox.