Martinez would be giving up a lot of size, and he probably doesn't carry a speed advantage over Ward, so I think a comfortable win for SOG. But really, it's not even fair, are we just talking about this because there aren't any SMWs good enough for Ward right now?
I'd have Ward by a fairly lopsided decision, but I'd think Sergio would have enough in him to keep the fight exciting. As far as Froch stopping him, while certainly a possibility, I can't honestly say I can really (clearly) imagine how that fight turns out.
I think this would be a very ugly fight aesthetically. Given the styles and stances of the two, I could see this one being stopped due to cuts from headbutts. In that event Andre would probably win on the scorecards.
Id rather see Chavez jr. or Golovkin fight Andre Ward. I wouldnt mind Martinez vs Alvarez at 154, Or possibly against Bute, Pavlik II, Abraham at 168.
All that being said, He will likely retire a middleweight.