They both mean the same thing IN ESSENCE the same as Because. Even in instances where one is more commonly used, the other could be substituted. As Chris pointed out 'Therefore' is commonly used in mathematical equations. For example, - 40 celsius = -40 fahrenheit, THEREFORE -40 fahrenheit = -40 celsius. If you switch it to SO it still works - 40 celsius = -40 fahrenheit, SO -40 fahrenheit = -40 celsius. http://dictionary.reference.com/browse/therefore

The word " so " is more common. The word " therefore " is used mostly in literature and prepared speeches.

It works in the sentence but the words are not exactly interchangeable. Using "therefore" here makes it seem like there is a bit more finality than "so."