both = this one AND that one either = this one OR that one Quite simple really.

Either one is X. Both are X. Both of these are X. Either one of these is X. X = an adjective (good/fine/okay/nice/fun) The above sentences all convey the same meaning. Grammatically, "both" is plural while "either" is singular. Also, like 雲小泉 wrote, "both" has a more inclusive meaning of "this one AND that one" while "either" has the more exclusive meaning of "this one OR that one", and either also often means that the two options are mutually exclusive (i.e.: you can only choose one of them, not both of them). For "you may call me or visit me, either way will do" using "either" sounds better and is more common than using "both." However, if you want to use "both" that is also widely acceptable in conversation.

You would not use both, but only one. Therefore, "either" is better.