Short answer:
Use IPv4 whenever you can, it is more effective and has less overhead.
IPv6 has only been implemented in case s/o faces a situation where IPv4 is no longer available.
Slightly longer answer:
Getting IPv6 to work is not trivial on Windows. IPv6 has a feature called “privacy extensions”, which is enabled on Windows by default (on purpose). This means it assigns TWO “externally visible” IPv6 addresses, one permanent one, and a temporary one that changes with each reboot. With “privacy extensions” enabled, ONLY the temporary one will be allowed to accept incoming connections. That means you can NOT properly use the permanent IPv6 address, which seems counter-intuitive, but this is indeed working as designed. So, for local servers/clients, if you want to use IPv6 (which, as pointed out above is completely non-sensical if IPv4 is still available), make sure to specify the temporary addresses, not the permanent ones. If you want to connect ppl from the internet, there is no NAT’ing in IPv6, only firewall configs in the router to allow data to come through. This is not port forwarding at all. However, in order to connect from the outside world to a temporary IPv6 address inside your local network, the router also needs to be configured to let pass “echo requests”, usually that means allowing all ICMP packages. And your router usually refers to the PC in the network with the permanent addresses, not the temporary ones. So that means that you probably need to manually add the IPv6 addresses for your local PCs, because your router does not even list them. Last not least, the Windows firewall also has echo requests disabled by default on IPv6, so you need to allow this locally on your Windows PCs as well. Another option to simplify all this would be to disable the “privacy extensions”, but that would pose quite some security risk (as ppl would now permanently be able to reach your PC, at least if the firewall is open. It is not “disguised” anymore like it has been the case with IPv4 NAT’ing).
All in all: IPv6 is more complex to setup than IPv4, and you HAVE to educate yourself properly in case you ever want to use IPv6. But, again, if you can use IPv4, there is no need at all to migrate to IPv6.