The majority of Waid's work tends to follow the same trajectory: starts with decent dialogue, good characterization, and some hooks that seem somewhat interesting. Then his plots get progressively more complicated and messy, his characters start to fall off track, then he gets bored and leaves. His work is almost always enhanced when he's collaborating with someone who can help with the plotting, be it Kingdom Come, The Flash or Fantastic Four.
Millar, in contrast, has certain "quirks" to his dialogue that is pretty hit or miss and sometimes keeps his characterization from staying quite on track, though he mostly avoids this by not writing characters in existing continuities; his weakest work, IMO, is stuff like Civil War, where he's supposed to actually stay "on-brand" for the characters he writes. His work also gets weaker as his runs grow longer, but he also seems aware enough that he hasn't done an extended run on anything in like a decade.
Overall, I'd say Millar is better at coming up with "big ideas", even if his execution sometimes falls flat; while Waid is better at the finer points of comics storytelling, but much less ambitious in his ideas.
As far as the person goes...I don't know much about Millar as a person, but Waid's reputation precedes itself, and not in a good way.
