He's better defensively, got a Conn Smythe under his belt, I find his offensive skills and attributes more valuable than Hossa's as well. He's also a year younger if thats makes a difference. In addition, as silly as it sounds he fits the team dynamic more. Lets face it Detroit has a buttload of Swedes, and thats the way they like it. If it makes any difference, I also beleive Zetterberg is willing to take less to say with Detroit, than Hossa is.
The only thing Hossa has over Zetterberg in my eyes is his injury history. Zetterberg has had some reoccuring injuries, which is obviously pretty troubling when considering this contract is going to be a long term one.