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Without knowing the scale it's hard to answer this. But in general HAProxy is THE layer to sit in front of web servers. From DDoS protection rules to a true set of metrics and a real dashboard (good luck w/ Datadog metrics for Nginx), it's purpose IS to be a highly available Load Balancer/Proxy.

Nginx in comparison is for me, THE thing to use as a web server. When it comes to being a single layer away from reverseproxying requests to apps running on an instance or static file serving with caching rules Nginx is purpose built for this.

Both apps have support for what each other do though. For the more pedantic it's easy to say "But x can do this too with this feature". Usability wise though, nginx and HAProxy are distinct. I went through the comparison very recently while testing setup of HAProxy as a load balancer in front of multiple webservers running Nginx which in turn sat in front of several apps running on each instance.

TL;DR - At a slightly larger scale (scale being number of instances + apps) HAProxy and Nginx are great to use together as opposed to one over the other.