It depends, really. Bendis and Millar are such drastically different writers it's hard to compare.
Millar is essentially an action-movie writer. Big explosions, hard-ass heroes, hard-ass dialogue, massive and brutal fights, this is what he does best. However, his politics far too often leech into his writing. As had come up in one of my discussions with Dread, the Ultimate X-Men were initially going to be a presidential task force until Dubya was elected. Then he scrapped his plans and went ahead with what he did anyway. Coincidence? Probably not. Ultimates 2 is overtly political, perhaps even too much. This can be overbearing for most readers. His other weakness is to have anti-climactic endings from time to time, particularly his "Anti-Hush" storyline in MK Spider-Man. Highly anti-climactic. And last but not least, Millar seems unable to write characters who aren't sarcastic *******s.
Bendis is a TERRIBLE superhero writer. Give him something very superheroic to write and he cannot do it, no matter what his ego and sales charts tell him. However, if you take away the superheroics and give him a more crime-noir/street level superhero story to tell, a la Alias, most of Ultimate Spider-Man or Daredevil, he really shines. His dialogue ranges from really solid (Daredevil) to irksome (Ultimate Spider-Man, the butt of many Bendalogue jokes). Put him in the right genre and he's great, but out of his element (Avengers, Ultimate X-Men, House of M) and he's absolutely awful.
All in all though... as far as track records go, I'd have to give it to Mark Millar. Volume One of Ultimates was fantastic, the best of the Ultimate stock thus far. Ultimate Fantastic Four under his pen has been superb as well. Basically, as long as he's in the Ultimate Universe, he's not half bad. In other areas, he's not quite as good.