haproxy tends to be lighter weight and higher performance. Also, haproxy gives (as this article mentions) a lot more information regarding status information, which can be used to monitor and manage the applications being load balanced.

Also note that Willy Tarreau has been a Linux kernel guy for eons and does a lot of upstream kernel patches, some of which haproxy benefit from. Most recent example: https://github.com/torvalds/linux/commit/3979ad7e82dfe3fb94a...

Willy also was the long term maintainer of the 2.4 Linux kernel (yes... 2.4), and does a lot of maintenance with the long term 2.6 release. So many embedded companies rely on his trees for their devices it isn't even funny.

I honestly don't know, I never made serious performance tests. The external exporter can make use of every variable that nginx is able to use in a log_format. I guess the internal Lua scripting has access to even more data.

Performance wise I assume the external exporter has less impact on Nginx' operation because it doesn't keep shared state or run code inside Nginx at all (see https://github.com/knyar/nginx-lua-prometheus#caveats), all Ngnix does is push out log records over syslog/UDP. If the exporter should overload that doesn't concern Nginx at all, what happens is that observations get dropped.