really depends on setup... Nginx is good for serving static stuff (jpg, gif, png, css, js, html) while varnish can cache all that in memory for really quick responses. It also caches dynamic content so in general it lowers the load and increases performance by far. It's bit complicated to configure...
I have some experience with both and if anyone need any help - feel free to contact me at celebempire{at}gmail.com
http://www.awmzone.com/services