Good question.
I just think that Millar's nothing special. YES, he's a good clubhouse guy, and YES, when he's hot, he's HOT, but when he's cold..... he's an almost automatic out.
It's my preference obviously (and some people feel differently, I understand that) but I prefer consistency, and defense. I believe that Mientkiewicz can help this team playing everyday MORE than Millar can with his defense, and decent offensive statistics.
My prediction from an everyday Mientkiewicz:
.290, 15HR, 70 RBI.
From what we've seen, an everyday Millar is:
.285, 20 HR, 80 RBI.
So, is Mientkiewicz's defensive advantage over Millar worth 5 HR and 10 RBI? In my mind, yes it is.