WAN IP of course, since it's the one that's public.
Well of course you
can't access using your WAN IP because you have not configured the NAT loopback, if your router is capable, but if others on the Internet know your WAN IP then they will be able to access.
Of course this subject has been well and truly covered in these forums many times before as a search would have revealed, see the links below to 'how to post' and 'how to search' forum announcements.
And of course you
can test your XAMPP from the Internet by using a proxy.
Of course you could even use a service like no-ip or dyndns, for example, to give public access to your XAMPP servers.
BTW XAMPP is not recommended for public access from the Internet as it has many security issues that are designed to allow developers little hindrance from using it at localhost. level.
Some useful reading for new forum members that may help you to find suitable solutions:viewtopic.php?f=16&t=48331viewtopic.php?f=16&t=32670
I will close this topic now as router issues are, of course, off topic and as you pointed out, your XAMPP installation is working perfectly.