Well,the answers other people give are all right.In the dialogue,different words make different reactions.primarily,in listener's attitude. if you use "tan",the listener will not immediately know the issue which you want to talk.good or bad? he may feel a bit nervous.he may think:"Oh ,it's not a joke,I must seviously treat it".in oral Chinese,we usually use reduplicated words.like,"tan yi tan","tan yi xia","tan tan". but if you use "shangliang",he may think:"dude,you want to get some suggestions from me.Ok,say!" similarly,you can say "shangliang shangliang","shangliang yi xia".but please notice that,you cannot say "shangliang yi shangliang".there is no such experssion in oral Chinese. Good Luck!

" shang liang " is more like " discuss, exchange opinions then make a decision for something". " tan " is more like " have a talk with somebody about something" to make the counterpart know something or get some information from him...

商量means exchange opinions （discuss） about some issue. 谈：sometimes has the meaning of 商量；sometimes just someone tells something to other person. For example, a teacher said to his student: 你过来下，我找你谈下。

same: Both tan and shangliang are all about discussion.tan usually uses oral ,shangliang is formal and shangliang is some people change their minds ,it's equal . tan sometimes uses in boss and empolyment,