Which takes priority, ipv4, or ipv6?

[Richmond <dnomhcir@gmx.com>]

I have configured an ipv6 tunnel. If I visit this site:

http://ip6.me/

The "normal" test shows my ipv4 address, and the:

http://ip6only.me/

shows the ipv6 address.

However if I switch my DNS from opendns to the one provided by my ISP
and then run the "normal" test it shows the ipv6.

The note says:

(preference depends on your OS/client)

So how is the preference determined? It seems to be determined by the
DNS, but why or how do I tell for example with host -v?