I think the J-class seats are likely to be quite tight during the 2 flights that you've mentioned 'cos as far as I know, there is a major medical conference going on in Chicago during that time.

Unless you have a good chance of mileage reward, I would not bother spending the extra money from L to K.

Btw, will you consider another route to Chicago? I think the chances of miles redemption upgrade or Op-Up is higher if you consider either YYZ or SFO as the first gateway.