Comment by zh

NginX is very good WEB SERVER - so it shines in serving content - web pages, caching images etc. But in proxy mode, the load-balancing between backends is pretty basic. HAProxy is born to be very good LOAD BALANCER - you can have different balancing algorithms, not only round-robin. HAProxy have also TCP mode, so you can balance your MySQL backends, for example. Also for me, personally, HAProxy stats rock.