id stick with the mrk3. Im hopefully making such a purchase soon, and the 6D is kinda cool spec wise, but I dunno, for me, the better build, the feel, the AF, 100 percent viewfinder and weather sealing is real nice. oh and dual cards is real nice. I would imagine that the video could be better. the extra fps would be real nice too. an extra added punch. as nice as the 6D would be especially from your xsi upgrade (im on a 40D still) then either would initially feel amazing, but id go mrk3. especially because its older brother, mrk2 had such a history.