[TimTheTinker]

I’ve never used either (just an F5 appliance), but I know HAProxy is built by a core Linux kernel contributor and has a reputation as the foremost infrastructure-grade software load balancer. In fact, a lot of hardware load balancer appliances run HAProxy under the hood.

From all accounts, if you really need a load balancer, or even if you just need failover, HAProxy should be your default choice. It has high-availability features, monitoring, supports TLS & SNI, HTTP2, session replication, Lua scripting, and almost any other feature you might need. It was also designed from the ground up to be a high-performance, high-availability load balancer. (NGinx does several other things.)

Stack Exchange (the company behind Stack Overflow) uses it in front of their IIS application servers, and so do a lot of other smart people.

Also, it appears NGinx has commercial/OSS conflict of interest issues — like in recent versions all monitoring functionality was removed from the OSS distribution.