Matter of personal taste, I guess... Both of them tell interesting, well-plotted stories, and I'll admit that Miller has a better handle on inter-character dynamics.
However, if we're talking entertainment value, and how much their individual books put a smile on my face, i'll have to go with Millar all the way.
Millar adresses grim subject matter with a healthy dose of dark humor, and a refusal to allow self-seriousness to creep into his work. He seems to write everything with tongue planted firmly in cheek (sometimes BUTT-cheek), and while that places certain limits on the kind of stories he writes, I find them eminently enjoyable.
Chosen -- until its final pages -- is a wonderful example of Millar being capable of doing strong, heartfelt, character-driven work.
Ultimates established that he can breath new life into staid icons, and give them an awe-inspiring larger-than-life quality, without ever losing sight of real-world social and geopolitical relations.
Heck, even for all its flaws, his UFF run has never been short of an exhilirating thrill ride.
In some ways, this is the antithesis of how Miller treats his stories, in which a sense of obsession and gravitas seeps through every page. It worked the first time around, on Dark Knight Returns. But after that, it all became a little heavy-handed and repetetive in tone, even if it was masterfully written. The man is clearly intelligent, articulate, and well-read, not to emntion a genuine lover of the medium, as Ourchair assured me, after he finished reading Eisner/Miller. But his work just strikes me as a little too, er, macho and angsty for its own good.