I've done a PhD, and quite a few other WWers are in the middle of one.
I don't think the overall ranking of the university matters much, as a) it's not a taught degree and b) like so many practice-led disciplines, excellent CW goes on in places which may in other ways not be that wonderful or high in the league tables. I really don't think that anyone wanting you to teach CW is going to be that bothered by where you did it in purely abstract terms, because they know that.
I'd suggest that the most important thing is for you to find the right supervisor and the regs which mean you can do the kind of project you want to do.
And there's a lot to be said for local - I live in London but did my MPhil at Glamorgan, which at the time couldn't be developed into a PhD (these days it can) so I did my PhD at Goldsmiths, which is up the road...
I don't know if you've seen this, but it might help you think through the issues:
http://emmadarwin.typepad.com/thisitchofwriting/creative-writing-phds-the-paradoxical-beast.html