I'll let others comment on the technical differences.
Have you read The Maglev paper out of Google? At the time right before that was published, one of my engineers was implementing much the same thing. They considered both HAProxy and nginx for the second layer of load balancing. The were technical reasons in either direction. There was some pressure (from me) to go with nginx because we were already using it as our primary webserver, so we could avoid introducing another new technology for people to master.
We went with HAProxy. Why? Because when said engineer contacted them, describing what he was doing, they (here: the main HAProxy dev) engaged in discussion, helped, even included his needs in their planning. At least at the time, nginx folks just responded with that they'd talk to him after he had secured a licensing deal. The uphill battle this engineer would've had to fight in corporate politics to get licensing sorted out that early in the prototyping phase would have been rough. Last I heard, the company still had a licensing/support deal with HAProxy.
I also had a great experience with HA Proxy open source support. We had some sort of config problem when we enabled H2 (IIRC, maybe it was going to 1.8 from 1.7). I did a bunch of testing of it, documented what I found and where I found it acting funny, and posted to the haproxy mailing list. A dev replied with something like "Sounds like you have this config option set wrong". Problem solved!
We had bought a ton of Fortinet gear to firewall+load balance for us, but in the end could never quite get it deployed. I got the haproxy set up instead and it's been amazing!
Can't tell you. Don't work there any more and don't remember in detail, but wouldn't disclose that if I was still there. Let's say: a significant fraction of an engineer per year. (But note that this was a very large internet company, worth tens of billions, to have had to build it's own multi level load balancer, not a startup.)
You can't spend money on haproxy license. It's free. They make money by selling appliances (servers with haproxy pre installed).
Nginx however is $1900 per year per server. There are plenty of critical features missing from the free edition, for example the status page to see available servers or metrics exporting for monitoring.
Just to do justice to my coworkers working on the ALOHA appliance, it's not just a "server with haproxy preinstalled" but a tight integration of haproxy plus a few management tools into a dedicated distro built from scratch and packaged as an upgradable image like you'd have on your routers or switches. The whole OS image is around 16 megabytes, kernel included, and it contains a 10 Gbps-capable anti-ddos module, a web interface, and troubleshooting tools. And of course you have root access on it and it doesn't void your support to start to hack on it (not pointing the finger at anyone, but still a little bit :-))
Have you read The Maglev paper out of Google? At the time right before that was published, one of my engineers was implementing much the same thing. They considered both HAProxy and nginx for the second layer of load balancing. The were technical reasons in either direction. There was some pressure (from me) to go with nginx because we were already using it as our primary webserver, so we could avoid introducing another new technology for people to master.
We went with HAProxy. Why? Because when said engineer contacted them, describing what he was doing, they (here: the main HAProxy dev) engaged in discussion, helped, even included his needs in their planning. At least at the time, nginx folks just responded with that they'd talk to him after he had secured a licensing deal. The uphill battle this engineer would've had to fight in corporate politics to get licensing sorted out that early in the prototyping phase would have been rough. Last I heard, the company still had a licensing/support deal with HAProxy.
Good presales matter!
(Edit: can't spell.)