讨论：discuss. could be any topics. 商讨：discuss, exchange views on major and complicated issues. for example: discuss economic cooperation between two countries (商讨两国的经济合作）or discuss investment plan between two companies (商讨两公司之间的投资计划）。 商讨usually will reach certain mutual understanding toward the issue being discussed。

討論 and 商討 have completely the same meaning. just small comment on that ： 商討，has more degree in writing form. the both not neccessary to reach any agreement in the end of discussion.

商讨 = discuss one matter 讨论 = discuss some matter