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.
EP, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS