My dream team without a shadow of a doubt, though Mignola's artwork is so caught in my mind as being atached at the hip to his writing (seems obvious but you know what I mean), that having Moore's words and plot instead would be strange first time round.
Still certainly a dream team and Moore is quite the fan of Mignola, and unless I missed Mignola's burning hatred for the bearded Northumbrian, I think its mutual. So a possibilty indeed.
As said Moore and Miller is a match made in hell. Miller's good at what he does but what he does is what Moore lampoons, attacks and inverts in his own writing. His testosterone laden heroes are certainly not Moore's cup of tea (his incarnation of James Bond as a shifty, backstabbing, would-be rapist says it all really). A gnostic high-concept Northern hippie and a New Yoiker pulp fiction writer isn't a good mix.
Also despite loving Miller's best work, I do dislike the fact that Alan Moore often gets loaded with the responsibility of ruining comics by making them 'mature'. This is partly due to the fact plenty of his work has been child-friendly adventure romps, but also that the vast majority of mediochre writers & artists who inhabit this new mature world of comics seem to take a lot more from Miller's ultraviolence. Again this isn't his fault, and there's plenty of pretentious pseudo-philosophy flying round thanks to Moore's influence but eh.
League of Extraordinary Gentlemen is the greatest thing ever and the VERY end of Dark Knight Returns sucks (well its a silly twist at the end of a great story at least) so I'm bias towards Moore over to Miller.