I'm not really sure regarding Frazier's personality, I've not watched enough Vikings games to see how he reacts on the sidelines. I'm also not really sure how much personality really matters, or if it does matter perhaps just as many players react to a coach that has "fire" as to a coach who doesn't show emotion on the sidelines. I do know Tony Dungy has been quoted calling Frazier "a leader of men."
Also, in my opinion Frazier's credentials are just as if not more impressive. He's been a successful head coach at the college level, has been a position coach and is credited with improving both the Eagles and Colts passing defenses, has a Super Bowl ring while with the Colts, and been regarded as a top defensive coordinator in the league. He played in the league as well for the Bears, winning a Super Bowl ring as a player and led the team with 6 ints that season.
As for Grimm, is he regarded as being a firey coach?