What division does Mirko have the most to gain by competing in?
205 right now has more prestige because it has a good champ and a handful of contenders along with a stable of decent fighters. Liddell, Ortiz, Jackson, Bisping, Jardine, Babalu, and Evans (he should fight at 205 or less).
206+ is thin outside of Vera, Sylvia, and Arlovski. Mir has lost two of his last three, Kongo got exposed at UFC 64, and there is no guarantee that Emelianenko's brother is on his way over.
Some say that Mirko will try and be a double champion at 205 and 206+ but I don't know if UFC wants to see each of those titles defended every six months (at 4 fights per year max, likely less).