==> connectivity includes one very important component IMO: the quality
of network connections. This is often ignored. It's a high risk for a
vendor to enable IPv6 by default, for example, if that'd result in lower
quality connections to e.g. dual-stack web servers (a very real fact in
6bone), this should be taken into consideration at least in the short
term.
i don't understand what you want to do about it. will you ping/ping6
destination and pick shorter roundtrip? when multiple A records
are returned for www.foo.com, do you probe them to get the optimal
performance? i believe the "quality" factor does not matter, and
it cannot be measured from endpoint (at least within short time).
I have wanted for a long time someone more competent than me to run an
experiment where one, after a name lookup, would send SYN packets to *all*
the addresses returned, and only complete the three-way handshake on the
first one to reply.