Even if I did some purely objective benchmarking I'm not sure I could answer that question because the configurations I've used in all cases are so drastically different.   I've changed my apache configuration constantly, but in every case the configuration and functionality has been very different than my lighttpd configuration.  Just using fastcgi w/ lighttpd, but not using it yet with apache skews things a bit.  I have several php-cgi process waiting w/ my lighttpd configuration, whereas with my apache configuration, mod_php is reflected in the size of my apache processes.

When I'm watching processes, my apache processes are running using anywhere from 5.7MB to 15MB of memory each (private memory).  Since I'm using the prefork MPM, with my low-traffic configuration I only keep a minimum of 5 spare servers forked, but it's pretty obvious that this setup doesn't scale well.

With lighttpd, I only have two processes running with my current configuration.  One is lighttpd and one is lighttpd-angel.  I won't explain what the angel process is as there's a full page document that explains it if you google it.  Anyway the lighttpd process runs about .9MB and the angel runs about .1MB .  Lighttpd has loadable has a few loadable modules similar to apache, and I'm using four of the optional modules.  A big difference here is that I'm using fastcgi so php processing is separated out.  My fastcgi configuration has 8 processes at 1.7MB each.  This is configurable and there are docs that explain how to calculate how many processes you should run to accomodate your estimated load.

So, my disclaimer is that I could certainly reduce the memory usage of Apache as there are still a few modules I'm loading that I probably really don't need.  I also plan to do more testing with the worker mpm and fastcgi on apache and although I've run the worker mpm before I can't recall what my memory usage looked like with that.  I'm not saying my apache configuration is optimal, but it was absolutely trivial to get my lighttpd configuration to a level of memory usage far below what my 'typical' apache conifguration uses.  I have a lot of apache modules loaded, but there are a lot that I have not loaded such as all the webdav modules.  There's a lot of functionality that my somewhat stripped down Apache configuration has that lighttpd does not, but none of it is anything that is required for even desirable for Piwigo, it's just functionality that I might use for other parts of my website.

I expect to move to move my main site to lighttpd very soon.  I think as soon as I decide on the php acclerator and test that I'll make the switch. 

Another thing I have thought about is that you could potentially serve different parts of your sight from apache and lighttpd if there was some advantage to that.  I haven't considered this seriously enough to investigate what kind of scenario would benefit from this.  Maybe if you had some specific part of your site that needed obscure functionality, you could use apache to serve that part, and then use lighttpd for everything else.

One other thing that's nice...my lighttpd configuration is a single file currently at 22 lines.  Speed, simplicity, minimal resource usage...I'm liking it a lot!

Just a status update on the lighttpd setup.  After getting my setup working great on Arch Linux, I decided that I still preferred Gentoo and went back to 64 bit Gentoo for my main machine.  I rebuilt the whole web environment on Gentoo very quickly and everything has been just as stable and fast.  I enabled a lot more functionality and the config file is still incredibly small and easy to manage.  I haven't even bothered to install Apache and I don't expect I'll have any reason to ever go back to it for my home setup.  Anyway, I just wanted to provide an update to confirm that this setup was stable enough for me to switch permanently, and I'm not just running Piwigo, but also Drupal and a few other web applications on this setup.

Vern