Draw from the experiences of what people who run much busier sites than many will ever run ... use the strengths of both. For example, Apache for front-end processing and have lighttpd behind the scenes streaming media, pictures, thumbnails, etc. I believe that is what Youtube does, at least for thumbnails from articles I've read in the past. Of course you can run completely off lighttpd if it provides all the features you need. I believe Pirate Bay, one of the largest trackers in the world uses lighttpd.