This page here compares nginx and lighttpd:
http://www.wikivs.com/wiki/Lighttpd_vs_nginx
There are numerous reports of memory leaks unfixed for years as well as severely worse CPU consumption (2% with nginx compared to 98% with lighttpd).
Do these problems still exist in the current stable version? (apparently 1.4.28 from the official website)
Best Answer
There is a recent side-by-side comparison of the nginx/1.0.6 and lighttpd/1.4.29 resource consumption and they both look good, albeit with different profiles (performances/resources):
The rather long tests do not show memory leaks (many charts included).