I haven't used stunnel with HAProxy, but I'm a long time user of Pound, in a few different environments.
Pound does a good job with SSL. It's dead easy to set up, uses standard PEM certificates, and passes enough SSL data back to the web server that your applications know what they're dealing with. In addition to
X-Forwarded-For, you get
X-Forwarded-Proto, so if you're running both http and https versions of a site, you can give your AJAX/XHR callbacks a URL that won't cause security warnings. Pound also supports client-side certs, wildcard certs and SNI. So there isn't much you can't do with it.
Also, I'm a little leery of putting up production services using tools that require a third-party patch to provide critical functionality. What happens when an stunnel vulnerability is discovered, and the change breaks your patch? Will you run insecure for a while, or run without X-Forwarded-IP headers for a while?
I've never had Pound crash. Except for a weird firewall-related bug that got fixed back in 2005, it has always behaved flawlessly for me. One of the things I love about it is the external control program it comes with, that lets me disable/enable services and back-ends, or view the current list of sticky sesion mappings, etc. As far as I know, haproxy doesn't have something like Pound's poundctl.
In addition to using Pound at work to load balance a cluster of five web servers, I use it on my personal server. There, I run web servers on virtual machines that use private IP addresses (FreeBSD jails). Multiple virtual web servers load balanced behind Pound running on the physical host. I suppose you could do the same with stunnel+HAProxy, but again, I prefer to install software with full community support, without non-standard patches.