Out of interest, do you have any metrics to support this?

It would be interesting to see data that compares the two, and see how close they come in different scenarios, and how tuning/configuration might affect the performance.

If you need just a generic cache, then using varnish, nginx or httpd are all fine choices. The issue is when you start scaling at obscene amounts of traffic. At that point, having a single single attempting to do too many things just doesn't work. varnish was/is designed as an extremely fast and performant cache which, when correctly configured, simply beats the pants off of nginx or httpd. It's easily as stable as nginx and httpd and as compliant as httpd (nginx is a bit less so).

For caches, the bugaboo is latency, and event architectures are not good choices when small-as-possible latency is an issue. It is here that heavily threaded architectures really shine. Sure, you can likely get by with less resources w/ events, but (1) performance will suffer and (2) as thread implementations improve, threading will only get better and more "stingy" re: resources.